I'm sorry. I'm in the same boat. While my sister doesn't have a girlfriend, she has had same-sex attractions for years, and decided to quit fighting it about a year and a half ago. She didn't try to use God as an excuse, as we were raised in a very fundamentalist Baptist church, so she's more decided that either God doesn't exist or he isn't someone she'd care to know. She seems to think that if I love her I will likewise cast off my moral beliefs, and she has some support in this from my family (I'm the most conservative one in my family). It's always tense when I'm around her because she has a way of forcing the topic to come up. In a lot of ways she isn't the person she used to be, since now she seems defined by and absorbed by her sexual orientation.
It's very difficult, as we were very close when we were kids. I have had to give up a lot of dreams of the future that I took for granted before--a brother-in-law, nephews and nieces. . .
Maybe we should start a support group for Freepers with gay family members. ;)