Now I'll talk about Gen 1
There is one sibling worth mentioning here: Vera. Lara's sister. I'm not sure where you'll met her, probably nowhere good, but I do know the quest to unlock her involved helping her out because she's being accused of murder. Probably rightfully accused but the case is she wants to change for the better. You COULD be a bad influence and unlock an evil path though.
Everyone else I'm going to mention are friends, not siblings of the Deadwylers. This means you can double dip also romance the Deadwylers along with. 😂
Preston: He's a celebrity so it's hard to get to him. He's probably found in one of those exclusive clubs, the kind where you'd also find Lady Seraphine. However, he's not picky. He's famous for being a womanizer. So the only hurdle is getting to him.
Darla: Also famous. Rarely she'll be presenting herself in the theater of the second town, where Ari sings, but usually she'll be in other towns. She may also be found in the exclusive clubs. You'll have an easier time romancing her if you've also gained some notoriety yourself.
Ione: Darla's twin. You can but shouldn't romance both. Nothing too bad would happen though, except only Darla will keep things going with you. You can find her in special events, using this 'magical contraption' that 'makes instant images of the moment'. Even if you don't romance her, you can still learn to use this contraption.
If you befriend any of them well enough you might get an opportunity to get invited to a private party (Preston is the most likely to do this invite). At this party you can meet Alan! If you've already met Lara then Alan will tell you that they're together. Or if you already started to romance Lara, Alan will tell you he likes her so you may get them together.
Check out the rest of the Romancing a Deadwyler series with the tag!