They don't "change" their sexual preference. They accept themselves for who they are instead of spending their life unhappy with a partner they cannot truly love or be happy with. If OP cannot accept that their parent (or any other person, for that matter) is perfectly normal to want to live with love, then they deserve getting cut off. The siblings can decide for themselves about OP once they are of age.

Nobody is saying they have to stay with a partner they don't love and be unhappy. And clearly they didn't.  It is just dificult for some kids to see their parent go for the opposite sex when all their life they had traditional sex parents. Some people just need time to get use to change. Sit down, explain the situation.  Tell them, "I know it's a lot for you to take in now, but with time you will see this is who I am and I will always love you. I will be here when you are ready to talk.  Not, "You didnt come to my wedding, You're homophobic! I'm going to cut all contact from your siblings with you!"  Wow so mature. But yeah, call them homophobic or bigot and tell them to die alone. Yeah that will show them how loving and inclusive the lgbt community is, well done.