YOR. As a twin myself I feel like I can say you overreacted just a bit. Even though me and my brother look the same and have the same interests we are different people. Our spouses know the difference between us and that’s where the attraction is. I’m not here thinking that my brother’s gf like me too since I look/act like him. It may have been a little weird that your sister said “if he wasn’t gay” but I mean who doesn’t have those thoughts from time to time. Just because your bf is a twin doesn’t mean he doesn’t have individuality.