I have always loved the concept of people flirting with Usopp and making Sanji jealous, because it's cute to see him deny his feelings until somebody else has what he is too afraid to go for. But it's even funnier if it involves somebody as cool and mature as, let's see, Sabo.
Imagine the coolest, most epic, nicest (which is what pisses Sanji off the most) guy shows up out of nowhere claiming to be your captain's brother and he is not only hot, but a gentleman. A true gentleman. Suit and all. Fancy hat and everything. Sanji knows Usopp likes hats, generally, he pushes away the need of buying a hat for himself now.
And he's got everything Ace had to charm everyone (it runs in the devil fruit, at this point) but also the maturity enough for you to feel safe and intimidated at the same time around him. He's playful and silly and Nami scolds him the same way she does with Luffy if Sabo gets too indulgent of their captain's decisions, but he still holds that level of importance Sanji feels overshadowed by.
So for Sanji it's great to see somebody take care of their captain the way his brothers never did with him, or at least, it should be. But he doesn't know if it's that specific thought what bothers him (the stinging sensation every time he sees the brothers together is something he can't ignore) or the way in which he's... Just nice.
Too nice. Too polite. And it's too much. He's too perfect. He's handsome, and warm, and familial and instantly can notice what you need. Sanji wonders how the hell this guy could ever be related to their captain, because he is, quite literally, the type of gentleman that shows up in Sanji's favorite fairytales. And it pisses him off, as much as he hates to admit it (but he does admit it, a lot, to Nami. She's so tired).
But, as much as Sabo pisses him off (for being too perfect and getting along with the girls so well, too) there's also the constant guilt behind it. This is Luffy's brother they're talking about. His only brother, now, too. And, sometimes, Sabo is too nice to him too. Helps him do the dishes. Offers him fire (which feels like a Deja Vu, and he kind of really hates it now, but feels completely different in a way). Actually listens and responds with a vocabulary and tone that makes Sanji think he's too fancy, but just sweet and almost-cute enough to make him feel awful about his jealousy. And Sanji can't let that pettiness get in between the brothers reunion.
Sanji does truly have to hold himself back from kicking him multiple times, though.
Because Sanji has this relationship with Usopp (that will-they-won't-they, we're best friends who kiss occassionally but don't know what we are, type) and he takes it for granted quite a lot.
So he doesn't feel that intimidated by Sabo's nature until he sees him clearly flirting with Usopp, and that's what makes any possible guilt fade away.
Nami says, when Sanji is clearly jealous and can't admit it, that he's genuinely just being polite. But Sanji knows. And Luffy does say something along the lines of "they're getting along" in a "he's MY brother, not yours!" type of way. Which means that if Luffy, his brother, notices the favoritism, it's not that much of a reach to assume something's going on there.
So Sanji tries to sneak into their interactions multiple times (despite Nami's advices not to) Getting just a tiny bit closer to Usopp. A tiny bit touchier than usual. Acting a lot cooler than he actually is, trying to match Sabo's style of gentle speaking. Usopp notices something's up, of course, he's not dumb. Sanji is always close to him but never this way, never around that many people. So it's a back and forth between Sanji and Sabo to get Usopp's attention.
Sanji swears he's not making this shit up. Sabo is obviously making heart eyes at Usopp. Placing his hand on his shoulder casually, as if they had known each other for a long time. Telling his own stories, feeding Usopp more and more ideas for his own. Acting so nonchalant, yet so careful with his every move. In a classy but careless way. He mentions Usopp burning down the government flag at some point, and how wonderful it was (his eyes screaming rebellion, making it clear that he is Luffy's brother). And Sanji laughs, about to make their joke about Usopp not being Sogeking. Usopp always does. But this time he doesn't, and it makes Sanji feel like a bit of an outsider when he was here first.
Not that Usopp owes him anything, but he doesn't owe anything to Sabo, either. Except for, you know, taking care of their captain and loving him unconditonally, but Sanji pushes and pushes that aside.
Sabo is definitely polite with everyone. But not this polite.
So Sanji bites back. Laughs louder. Uses the advantage of already being close to Usopp, to be closer, in a way that if Sabo did it, it would look odd. He smokes, because he knows Usopp likes it (and hopefully can light up the cigarette before Sabo does anything of the sort himself) and offers a drag to Usopp just because he likes the warmth afterwards (and sort of, the flame reflecting on Sabo's almost unsettling eyes, too). The thrill of making Sabo's calm state falter for a second is enough to leave him satisfied with himself for a while, too.
Usopp isn't complaining because he truly isn't getting what the hell is going on half the time, but that's exactly what bothers him. In his eyes, Sabo is actually just being polite. And he is, actually. Sabo genuinely just wants to get along with his little brother's crew and that's it. Or it was, at first.
But Sabo is a writer, and Usopp has stories, and it was just inevitable for him to wish to be closer. Riling Sanji up is just a fun plus.
Usopp just thinks they're both starting to get a tiny bit annoying (until he reads Sanji clearly, like an open book, as he often does, and realizes what truly is going on).
At some point, Sanji ends up believing what the others say. That Sabo might just be polite, he's just charming by nature. Which still bothers the fuck out of Sanji because, well, he's always trying to be that way so how the hell does he do this so naturally. He wants to be that way too. Make Usopp blush without thinking of everything beforehand (he doesn't realize his rizz works when he's just being naturally nice).
Then, Luffy interrupts them in the middle of one of their "Oh, yes, Usopp, tell me more about that story that's definitely true and happened but let me be just a tiny bit closer than this other guy", and Sabo always has to put his brother first.
Not without saying goodbye with a tilt of his hat towards Usopp (which makes Sanji's blood boil, btw) and a wink directed at Sanji that only he interprets as a threat (which it is. A dare, really. He's got the same flame as Ace in his eyes, but way more chaotic despite how gentle-looking he is).
So if Sanji blushes in the end, he tells himself it is out of pure anger and frustration and nothing else. And if Usopp casually mentions the fact that he likes Sabo (because he knows, already, what's truly going on), Sanji suppresses the need to ask him to like him more. And if Sanji spends all the dinner prep thinking about the two of them together, earth tones and revolutionary blues against his own stare... He can't help but putting way more care and love into Usopp's meal, as he always does, and way more in Sabo's too, just for the hope of another reaction he tells himself he doesn't enjoy.
