My favorite backstory at the moment is the one for my vampire sim, Caitlyn. She lived in San Myshuno with her alien brother Apollo, his wife, their three kids, and her own daughter. She was orphaned as a child and was raised by her much older twin half-brothers alongside their siblings, one of which was Apollo. So technically, she has no relation to Apollo, as Caitlyn and her half-brothers shared a father, and Apollo was born from the twin's mother. But they were raised together, so I consider them siblings even if the game doesn't.
Anyway, her story is that, because she showed up in their lives kind of late, she never really found a way to fit in to the family that well and was shunned by everyone but her own twin brother, her half-brothers, and the family dog. Once they were young adults, her brother left the house, essentially abandoning her, and went to live on his own in a different town. Caitlyn remained at home with Apollo, who was the only other one to not move out. When Caitlyn was a human, they had an okay relationship. They weren't friends, but they didn't hate each other either. At some point early in her YA stage, Caitlyn met Lilith Vatore, who turned her into a vampire. Pretty much as soon as her family found out, her relationships with them started going south, except with Wilson. They have always remained best friends, even after she turned him into a vampire against his will so that he could never leave her alone again. Even Apollo's toddler children didn't like her and she has a rocky relationship with her own daughter; they are friends right now, but not by much. Apollo dislikes her very much, as does the rest of his family. The last time I played Caitlyn, she had just moved out to get away from the toxic relationships in her life and moved in with her brother, Wilson, to help him work on his vampire skills, leaving her daughter behind for Apollo to look after.
I love playing her household because of all the drama, but I don't think they'll be as much now that she's moved. I'll probably focus more on having her her become a supersim, as, since she's immortal, I don't have to worry about her dying before she's completed everything. But I look forward to seeing where her story leads next.