I can't deny that rewriting parts of chapter 4 to accommodate William and Leia beings twins is a challenge, but given enough time, I think I'm up to the challenge. Here's my vision for it: William and Leia are the same family, taken in by the Zanes when their parents were murdered (that part doesn't change). But the reason for William bailing years ago is still because of the relationship with Leia, but not because of some "mob family" bullshit rule, just because...they're siblings...and because of "family" pressure. The forced mariage rules becomes a way to try and break that relationship at its roots, to distance Leia from William. So the whole mafia stuff will be there to stay, sorry if that still hurts your trying to make sense of it all
On a sidenote, I do believe that's what Ocean had envisioned himself at some point, when he said that only Leia and William would be related. When you have a closer look at the script and dialogues you do realize that stuff was probably the last to go in his numerous changes to family dynamics.