it's pretty simple - their parents grew up in ash harbor and so did they. their mom is from old money, but she didn't take any so they were kind of middle class growing up except for when grandma dropped off the big checks at christmas and on birthdays. they had brother first, then flo and then the younger sister.
brother should be at least 29, but anything older than that works. i imagined they're close but not the kind of siblings who tell each other everything, maybe fought a lot growing up but have each other's backs over everything. she never told him who the father of her kid is, but he threatened to kick everyone's ass that looked at her funny when she was teen mom-ing it. so yeah, he'd be an uncle to an 8 year old and besides that, the floor is yours. any of the relationship can change if you'd prefer.
he'd have two cousins in game ~kinkerbell (from the rich maternal side of the family - she has two older brothers) and ~coppery (paternal)