Curlfeather canonically groomed Splashtail, whether some fans want to admit this or not (looking at a redditor i got into an argument with). I did an analysis on this in the past, but TLDR; Splashtail became more radicalized than Curlfeather as a result of her twisting his "ambition" (fuck you Erins)
so, in the AU:
Curlfeather swoops in.
Trying to make sense of the canon timeline here; Splashtail and Fognose were born sometime before Lost Stars, while Curlfeather gives birth to Frostdawn, Mistpool, and Graysky in The Silent Thaw. So Curlfeather would've been in the nursery with Havenpelt while Splash and Fog were kits.
OKAY SO
In the AU, Curlfeather notices that Splashkit is… strange. It's not on purpose. She wasn't actively observing this child; she could care less about Havencloud's kits. But she's going to be a mother for the first time, and she's nervous because Jayclaw actually dies BEFORE Curlfeather's kits are born, not while they're young like canon. She's going to be a single mother, and Havencloud has promised to help her since, yk, at least Havencloud has Sneezemuzzle.
Both of Curlfeather's relatives, her brother Podspring (canonically Podlight) and her mother Duskfur (name will most likely change), don't like children in this AU. Duskfur has offered to babysit after Curlfeather begged, and Podspring told Curlfeather to go to hell. She's alone in the nursery, but she can get some tips from witnessing Havencloud with her kits.
When they are born, she notices that the dark gray tom is just a tabby. Nothing unusual, but as he grows older, he suddenly has a white face marking. How odd. And suddenly he's staring into nothingness, and he doesn't like being around the Clan, and from snippets she can hear from the nursery, they think he's odd.
Curlfeather doesn't have any major interaction with Splashkit. She doesn't step in when Sneezemuzzle and Havencloud neglect him and Fogkit. She's too focused on her own kits.
BUT the major interaction comes from before Softripple's death. Frostkit, Mistkit, and Graykit are born now, and she's doing quite well, but she often takes breaks because Frostkit has her father's eyes, Mistkit has her father's gentleness, and Graykit has her father's determination. It gets to be a bit much sometimes, along with being a first-time mother and whatnot.
Nursing feels weird, yada yada. And now she's alone, because Fogkit and Splashkit have become apprentices, and Havencloud doesn't have to be in the nursery.
So she takes a break, leaving her kits in the care of Turtlestalk (canonically Mosspelt). She overhears Splashpaw admitting to Softripple that he can see ghosts. Softripple, as I said in a different post, didn't fully buy into it. But Curlfeather believes it enough to keep an eye on the kid.
Still, she doesn't approach him.
Until after Softripple's death, and Mistystar bending to the impostor. Who does that? Not a RiverClanner.
Now, in the AU, Curlfeather and Podspring are actually born ROGUES because Duskfur was a rogue. They have no father because Duskfur fucking murdered him but yeah, they were born rogues. Dusk brought little Curl and Pod to RiverClan when the two were apprentice-aged.
BUT ANYWAY yeah, Curl was resistant at first, but she eventually integrated into Clan life and was named Curlpaw and then obviously Curlfeather. Her name MIGHT change to Lightfeather, but for now it's Curlfeather.
So back on topic, Curlfeather keeps an eye on Splashpaw, and sometime after Softripple dies, she notices that Splashpaw often pulls away from Minnowtail, wanting to be on his own. So, she approaches him while he's on a lonesome brooding moment.
Splashpaw doesn't want to talk at first, but Curlfeather doesn't really care about what he wants.
She asks if he can really see ghosts. He says yes. After some idle conversation, Curlfeather asks "Do you ever see StarClan ghosts?"
This causes Splashpaw to freeze. He never thought about this, ever. But now that he's thinking about it… "Well, no, but they'll see me eventually."
"Why eventually? Why not now? Aren't cats who can see ghosts special? Why do you think medicine cats exist?"
Splashpaw has nothing to say to these questions. They're ripping him apart, picking at his worst insecurities. And Curlfeather has him where she wants him. His reaction basically confirms that yes, he CAN see ghosts, but it'll be their little secret once Curlfeather really digs her claws into him.
And this also assists with her plan to become the leader of RiverClan. Curlfeather doesn't want to become leader because of malice, she genuinely thinks Mistystar is a terrible leader who makes awful and sentimental decisions that ultimately affect her Clan. Why did she apparently stay with Blackclaw for so long when he treated her like utter shit? She's weak and affected. Curlfeather loved Jayclaw to StarClan and back, and the stars knows she misses him everyday, but she'd never let memories affect her.
Curlfeather could never kill Mistystar. She'd have to get through all of her lives, firstly.
But Reedwhisker, he's doable. He's the deputy, and Mistystar's son, but all of her other children died anyway. Stonefur died. Bluestar died. It's fine. Curlstar sounds like a cool name anyway.
But still, Curlfeather doesn't want to dirty her paws directly, she still has kits to raise. But this kid, this kid who looks into hell could, with a little shove. Curlfeather spends more time with Splashpaw, planting these seeds into his head, promising that he'll be deputy when she becomes leader. Minnowtail doesn't question this. She tried to bond with Splashpaw so many times but couldn't, maybe Curlfeather could. It'd help him process his grief for Softripple, making new friends and whatnot.
Curlfeather is fully committed to her plan at this point with Minnowtail's "permission." She has Splashpaw, and she has her kits, perfectly moldable.
And by the time of the first book in ASC, River: Frostpaw is a medicine cat apprentice, Graypaw is a considerable enforcer, Mistpaw is shaping up to be a lovely warrior, and Reedwhisker is dead without her laying a single paw on him.