If the guys had little sisters they grew up with (AGSZC)
(inspired by @strayheartless ' post about Aerith and Genesis being siblings)
Angeal's little sister would be great at baking and theft. Yuffie would get into a petty rivalry with Angela over who could steal more from the Rhapsodos estate, especially materia. Angeal would not know this, and only see Angela's sweet, motherly, pie-baking side. Angela would also be a science whiz and use her theft to fuel experimentation.
She would get Angeal to try so many concoctions and he would always cave because of her big brown eyes.
Genesis' little sister would be Angry (tm) about how she and Genesis are treated by their parents, especially about how they treat him like an investment property and her like a political pawn. She would help Angela steal at all times.
Genevieve would be musically talented, and Genesis would be constantly trying to cast her as the maiden in his plays or getting her to write scores for his plays and poetry.
Genevieve would know how to act in any polite society...and actively choose when NOT to use that skill. She would inherit her mother's ability to insult someone while sounding kind. She'd inherit both her parents' ability to scheme and make money and political power. Her ultimate goal is to buy Genesis' freedom from Shinra one way or another, and secure her own freedom from marriage.
Genesis LOVES to complain about and insult her to her face, but he'd kill everyone in the room if they hurt her.
Sephiroth would take one look at a biological sibling and either shut down completely or latch onto them like he does to Cloud. I'm picking the latter.
Hojo would be dead, Miniroth would go on the run, and Sapphire would poop on him and he wouldn't know what to do until a kind, yellow-haired woman with her own baby took him in.
Sapphire would grow up tough, quiet, and cold to everyone but her family and Tifa. She is so silly around them. She'd be an expert hair braider and a fearsome hunter.
Sephiroth gets really heckin annoying every time she starts becoming more independent. Her first steps away from him? He runs around in front of her. Her first "no"? He says "no" right back.
Zack's little sister is a hugger and no one can escape. Cloud and Sephiroth jump out of their skin like cats when they hear her coming, and have to brace themselves. And they always hear her coming, she is LOUD.
She gets along swimmingly with Aerith and can keep pace with Zack on eating contests. She's very comfortable with dirt, blood, adventure, and farming. She's the girl you call when your goat is giving birth or your grumpy yellow boyfriend won't talk to you because he thinks you hate him.
Zara will tease all Zack's friends into doing what she thinks is best for him and them and no one will be able to resist her. She's usually covered in dirt.
Zack LOVES giving the shovel talk, but always fails because he either ends up best friends with her partner or punches them within minutes of meeting.
Cloud's little sister kinda just does her own thing. She's quiet, hardy, skilled, and not participating in anyone's BS but her own. The sibs pretend not to notice or care about each other's comings and goings, but they'll show up for each other armed to the teeth in a heartbeat.
Claudia Jr. and Cloud do have their share of knock-down, drag-out fights, but they always wind up being fine minutes later.
She's more likely to give Cloud's boyfriends the shovel talk than the reverse. He just sits there cleaning his sword and saying nothing. He's doing it because he feels awkward, but it comes across as extremely menacing.
Their idea of a good time is cowboy camping in the Nibel mountains during dragon mating season, because the dragons shed more scales then and they fetch good money.
