Stay away from the whole situation. It is a sign to your sister that she is up to satan's work. Keep her in prayer. No congratulation cards, no gifts and above all even stay away from their house, and you don't invite them into yours. It is a sign to them that you are slowly accepting them if you do have them in your home. Don't go to any of their invites either. You honestly can not say that you would enjoy their company. Hold Fast to your Faith.
My friend had to say no to her only son when he married a jew in the ѕуηαgσgυє. She fought within herself and her husband. When the day came she was not there. She is happy for keeping the Faith and her word that she would not attend.
My cousin has a grand daughter that has a lesbian and when my cousin's son invites her over to his home and my cousin is there she picks up and leaves. Her son scolds his mother(my cousin). How very sad!
Whatever you do, stay as far away as you can. If you are of the New Order (not church) don't listen to anyone who says that you can not be judgmental. Christ told us that "you would know them by their fruits". You judge by the outward signs and you have plenty of Red Flags.
How else will your sister learn. So show her signs that you are praying for her conversion from satan and his works.
You will not regret your decision to stand for truth as Our Lady stood by the Cross knowing that His Precious Blood would be in all the sacraments to save man. That is Love.