My reason for not wanting to go is still quite simple. God says no, so I have no choice but to follow my convictions and say no as well.
Michael,
First of all, I understand your reasons for not going, and your convictions. However, have you thought about it from your brother's perspective? He is lost and he wants his brother, who I'm sure he loves, even though he is behaving very antagonistic toward you, to be at an important event. But perhaps he is acting out because he is hurt or afraid that you no longer love him.
I have learned that the reason people behave in anger or spite, or other hateful ways, is because they are in pain (whether they realize it or not), and they don't really know how to deal with it. Fear, anger and rage are what comes out. Jesus has taught us to turn the other cheek. You know better than he does, and you know (I hope) the peace and love of Jesus. I urge you to extend that toward him. The best example a christian can give is not talk, but action. Of course I don't know the tone and manner in which you approach this issue, so I'm not going to assume either way, but hope that it was gentle and kind.
I completely agree with your stance of not being part of the wedding party. I don't 100% agree with not going to the wedding at all, but that is definitely your decision and I can understand your reasons and convictions for not wanting to go. Someone has suggested perhaps having your brother over a different day to spend time with him. I think that is a great suggestion. It shows him that while you don't support his lifestyle decision, you still love him as a person and as a brother.
Sometimes we let our carnal type of habits take over, i.e. impatience, superiority, anger (I do it more than I'd probably be willing to admit). We sometimes forget that we are ambassadors to those who don't know Jesus. A kind word turns away wrath. Yes, your brother is homosexual and is living a sinful lifestyle, but he is still a person who Jesus loves and wishes to come to the knowledge of the truth. The opposite of homosexuality is not heterosexuality; the opposite of homosexuality is holiness. If we push away those in that situation (not saying you have) how can they ever come to know Jesus? How can they ever have a chance to get anywhere near repentance and holiness?
I for one don't see homosexuality is "THE BIG SIN," but another on the list of sins, of which all are the same in God's eyes. All sin destroys us from the inside out and all sin seperates us from God. I have a few friends who are gay. I love them. It really saddens me to see the way that they're treated by Christians. They're vilified. They're made to feel like they're monsters and not worthy of love. That is the worst thing that a Christian can do to someone. I don't agree with their lifestyle and I can honestly say that I don't know what decision I would make if I were invited to a gay wedding; I can't really know until I'm in that situation. I just wanted to make this post to you to perhaps help you see a different perspective.
I agree 100% with WillieT; PRAY about this and ask Jesus what you should do. If you feel convicted about not going, great. But let Jesus be the decider, and not stubborness.
May God bless you in this decision, and help you learn the best way to repair your relationship with your brother!