So a very good friend of mine @dialga64bitz has created the Reverse Rock Sibblings AU.
The premise is the fateful day, instead of Barb finding Branch, Branch finds Barb. They encountered some sort of beast and Branch finds Barb injured and brings her back to the village, believing her to be a grey troll.
Barb kinda immediately latches on to Branch as a sibling, which makes her not want to leave. Also preventing her from going home is that she believes her dad may have died in the attack, and going home would cement the reality. In Pop Village she can pretend he's fine, but if she goes home and he's not there... then she knows.
King Peppy isn't happy with Barb being there, which is fine by here because she doesn't like him either. Because of her, Cooper realizes he's not a Pop Troll very early on, and resolves to go find his family when he gets older.
Barb lives with Branch in his bunker, and after some time she kind of begins to like to snack pack.
Cooper: She befriends him first, and he asks her about different tribes.
Smidge: Her and Barb are work out buddies!
Biggie: Not a big fan of him exactly but she likes Mr. Dinkles.
Satin & Chenille: They tried to make her rock clothes to varying success. They see it as an ultimate fashion challenge.
DJ Suki: They chillax together.
Guy Diamond: Honestly she can't stand this guy but she can respect his confidence. When Tiny is born though she immediately names herself Aunt Barb and dotes on him.
Poppy: They have a very up and down relationship. She appreciates that Poppy genuinely tries, but Poppy's tendency to not listen or do things the Pop Troll way caused them to butt heads often.
The night of the 20th anniversary Barb leaves the bunker to go check on the party, only to arrive just in time for Chef to appear. Due to Barb, the Pop Trolls were all able to escape being caught. Only a single Troll was caught by Chef...
Barb is captured by Chef and taken to the castle. She avoids getting eaten by Gristle due to being a bit too hard to swallow (she's not making it easy) and Chef listened to her for long enough to know exactly what to threaten Barb with.
Sell out the Pop Trolls and she'd spare her and her brother. If she didn't, she'd do something far worse to Branch and make Barb watch.
Barb reluctantly chose to save Branch.