This is giving me many many brain worms. I originally was going to put them all in the tags but it was getting so long that I have to put them in here.
Shen Yuan glares DAGGERS at his Qing Jing Shixiong during the next Peak Lord meeting after he takes his baby bun as a disciple because this is cutting into his Father-Son Make-Up Time (and he's got more than a DECADE to make up for! That's not much to a centuries' old immortal but Shen Yuan still knows that it's a Big Ass Deal to his son because that's all the time he's ever lived). Qing Jing Shixiong is, of course, completely unapologetic because he could tell immediately that Shen Jiu had the makings of a peerless scholar in him (just like his baba, Qing Jing Shixiong thinks bitterly) and was not about to let a Shen slip through Qing Jing's fingers a second time.
Normally Shen Yuan has a good and close relationship with his Qing Jing Shixiong as Shen Yuan is very scholarly minded despite being more invested in beasts than the Four Arts. Shen Yuan's Qing Jing Shibo had tried to poach him from the Beast Peak numerous times as a disciple (Shen Yuan is, of course, oblivious to this) but not only was Shen Yuan's Shizun (the previous Beast Peak Lord) wholly unwilling to lose out on his future head disciple, but Shen Yuan was solidly in love with the Beast Peak and became an unskippable cutscene whenever his hyperfixation was triggered. Still, Shen Yuan spent a decent amount of time on Qing Jing with his shixiong, who was strangely insistent on practicing alongside Shen Yuan, leading him to be at least passable (by Shen Yuan's standards) through osmosis.
(This is a long way of me justifying Shen Yuan still knowing how to play the guqin solely so he can play modern songs for Shen Jiu on it)
Shen Jiu, meanwhile, ends up in a quasi-split custody agreement between his Baba and his Shizun. And for his part, Shen Jiu thrives on Qing Jing without any harassment from his rich-kid shixiongs. There were attempts at first, spawning from jealousy over how much attention Shen Jiu got from two separate Peak Lords (namely Shizun) and how Shen Jiu took to the Four Arts and cultivation as a whole like a fish to water, outstripping his shixiongs in skill and power with blinding speed. Buuuttt they learn VERY quickly that bullying Shen Jiu will trigger Mama Bear Shen Yuan to swoop down on them like death from above, and for all his kindness and gentle hand towards discipline (compared to his martial siblings), Shen Yuan is ruthless when it's his precious son being harmed. Needless to say, any bullying of Shen Jiu is put to a swift and brutal end, and Shen Jiu's would-be bullies are forced to sit and stew in their jealousy, or worse, actually work to improve themselves.
Whenever it's Shen Yuan's turn to have Shen Jiu on his peak (Shen Yuan and his Qing Jing shixiong have something of a 50-50 custody agreement), he tells Shen Jiu of his uncles, his aunt, his grandparents, and of Shen Yuan's own childhood. Since I'm an adherent of Chronically Ill In a Past Life!Shen Yuan and Shen Yuan only ever talks about his family in the past tense (to say nothing of the small memorial altar in one of the back rooms of Shen Yuan's house, crowded with no less than five carefully carved tablets that Shen Jiu found when snooping one day), Shen Jiu concludes that Shen Yuan's parents--his grandparents, Shen Jiu thinks with no small amount of awe--sent his Baba away to Cang Qiong in order to cultivate to save his life. Baba may have tried to minimize and gloss over the severity and extent of his childhood illness, but Shen Jiu is no fool and by now knows that his idiot Baba does not show himself the same care he shows to others (despite his Baba regularly claiming that he's lazy while being the most hard-working man Shen Jiu has ever met). And naturally, his Baba flourished and became an immortal cultivator who stood above the rest. However, the family that had loved his Baba enough to send him away remained mortal and lived and died as mortals do long, long ago.
And in a way, Shen Jiu is correct. There's just no way he could have ever known that Shen Yuan is from a world far removed and alien to the one Shen Jiu was born in.
Even as a disciple, Shen Yuan had been tight-lipped about his origins, only ever speaking vaguely or deflecting any questions completely. Mostly this is because Shen Yuan's origins in this world were unclear even to him. After reincarnating, he found himself in a child's body in the midst of a smoldering ruin. No bodies had remained untouched by the long gone flames save for his own and the ruins were damaged so severely Shen Yuan couldn't decipher anything of use from them. Left with no other option, Shen Yuan had walked away into the wilderness, discovered what world he'd been reborn into, agonized over whether or not to join Cang Qiong before concluding it was the safest option for an apparently orphaned child in PIDW after a near miss with slavers, making it to the disciple selections and being picked by the Beast Peak Lord of the time (after a dead-heat race to claim Shen Yuan between most of the present Peak Lords), and then promptly cursing out the universe for placing him well before PIDW's canon once he realized he recognized NONE of these Peak Lords.
Yeah, Shen Yuan kept his past--both from this world and his previous one--close to his chest. But that just made any crumb of lore he let slip so much more valuable. Because Shen Yuan wouldn't be Shen Yuan if he didn't wife-beam nearly everyone he ever interacted with. Unbeknownst to Shen Yuan, he has a line of admires long enough to circle the whole of Cang Qiong several times. Nearly all of them are men, which goes a long way to reinforce his obliviousness of their affections (Shen Yuan's Qing Jing Shixiong is one such instance and he is a source of fierce jealousy among Shen Yuan's numerous suitors because he gets to live out a familial fantasy through the split-custody arrangement over Shen Jiu).
Shen Jiu doesn't mean to let his Baba's Lore disseminate to the wider world, but he admits he may have been careless when talking about it with Qi-ge. Some nosy busy-body overheard and Shen Yuan's suitors were fed well.
The news that Shen Yuan had a son definitely caused a stir, and naturally Shen Yuan never mentioned that Shen Jiu came from an unexpected encounter with a Spring Day Heart Lotus aside from his martial siblings (who could largely be trusted to keep their mouths shut). This coupled with the Lore Drops overheard from Shen Jiu spawn a number of heart-wrenching tragedies of a third son from a middlingly rich family that suffered from an incurable illness but was kind and loved (and beautiful beyond compare). And because he was so loved, this tragic beauty was tearfully sent away to a cultivation sect in the hopes that cultivation would be the key to saving his life. And it does. Too well, in fact. Because the tragic beauty out lives his beloved family by many many years, leading him to close off his heart in his filial grief. At least until he discovers he does still have family left. A son he never knew he had who was alone in the cruel world and suffering terribly at the hands of a monster (Qiu Jianluo is often cast as Secretly A Demon because the wealthy human sponsors of these works don't want to confront the idea that One Of Them could do something like That to Shen Yuan's precious son--to say nothing of the thousand and one casual cruelties they inflict on people they see as Lesser every day that would make Shen Yuan despise them--so the perpetrator MUST have been secretly a monstrous demon all along. The motive in this take for Qiu Jianluo's cruelty is often covetous and ruinous lust for Shen Yuan taken out on his son).
Shen Jiu's exact origins as a Plant Baby aren't really known outside of Cang Qiong (Shen Yuan's admirers amongst the mountain don't want to give the competition Ideas on how to baby trap him), so these stories often include a Generic Female Lead to act as Shen Jiu's mother. Characterizations of the supposed mother vary from story to story. Sometimes she's a scheming vixen who takes advantage of the tragic beauty's kindness and cruelly abandons both the beauty and the child they begot together when it no longer serves her. Sometimes she's the tragic beauty's First True Love who tragically perishes after giving birth but before she could tell him of his child's existence (somewhere, somewhen across the universe and sealed beneath a mountain, Tianlang-Jun sneezes). Sometimes she's the author or sponsor's blatant self-insert (often genderbent) who becomes the tragic beauty's True Love and wife and mother of his child who is cruelly stolen away by a covetous and cruel demon (Qiu Jianluo, who is spinning in his shallow grave). The last one is often something of a retcon as after the Tragic Beauty Male Lead discovers and reunites with his long lost son (usually thanks to cosmic chance or divine revelation. One or two include a Childhood Friend of the lost son making the reveal, but that's very rare), the stories veer off into dogblood dramas and blatant wish fulfillment that caters to one or more of Shen Yuan's many many suitors.
Shen Jiu discovers one of these stories years later and is furious that people are lusting after his Baba and writing such garbage about him that they share with the wider world rather than keep to their rotten selves. When Shen Jiu tries to tell his Baba about these affronts to his honor, not only does Shen Jiu find out that his Baba knows about these stories, but his Baba has been an avid follower of a number of the worst, verbally shredding these terrible stories as he resurrects his inner Peerless Cucumber while also being blissfully unaware that these dogblood garbage stories are all about him and written by and for his many suitors.
Part of Shen Jiu wishes his Baba would ascend with his martial siblings if only to hopefully get him away from his many depraved suitors, but the more selfish part of Shen Jiu wants his Baba to stay with him. He's quite pleased to find out his Baba feels the same, holding off on ascension to stay by Shen Jiu's side (which totally has nothing to do with Shen Yuan being confronted with the knowledge that his precious bun of a son is supposed to be the Scum Villain and the fate Shen Qingqiu suffered in PIDW doesn't look as vindicating as it did now that he knows it would happen to his son, whom he loves).
Unfortunately, Shen Yuan doesn't stop attracting flies after retiring as a Peak Lord when his son's generation ascends to the post.
The most dangerous amongst them being the Heavenly Serpent General, Zhuzhi-lang, who the Saintly Ruler of the Demon Realm is shamelessly encouraging in his attempted courtship (Tianlang-Jun is an avid fan of the stories Shen Yuan spawned and while he's too in love with Su Xiyan to seriously pursue Shen Yuan himself, he is enough of a shit disturber to goad his smitten nephew on instead). And maybe through the power of unintentional double dates Shen Yuan gets literally roped into, he susses out the Old Palace Master's schemes and averts Tianlang-Jun and Su Xiyan's tragic fates...