Well, the catch is that genetics are sneaky. 🙂 Sometimes the genes one has are not what's being expressed by one's closest female relatives - for example, my mother is a 34B (but she stays very skinny) and my sister is 36D, but I'm guessing I got my breast genes from my father's side of the family, where his sisters are all DD and up. 3 years in and I'm 38D-42DDD depending on the bra. (Yeah, I think some bra manufacturers are nuts, frankly. That's a heck of a range.) I haven't done anything but take estrogen, and not be skinny, heh.
I also know cis women whose sisters and mothers are small, and they ended up huge, or vice versa...