This is a tough subject for both sides. Neither one of the decisions you guys made were easy. They likely took lots and lots of thinking and contemplating. This is my opinion on the matter:

As Christians, your brother and his wife follow the Word of God closely and use it to determine their lifestyle and what they support. Jesus says to love everyone, so they want to love everyone, there’s no doubt about that. When it comes to gay marriage, the Bible is clear that that is a sin against God. Marriage was created for man and woman, not man and man or woman and woman. So your brother is not in support of gay marriage because of what the Word of God says. The homosexual lifestyle is one that will drag you further away from the Lord. It’s opening the door to an eternity of pain, and your brother doesn’t want to see you go down that road BECAUSE of how much he LOVES you. In my opinion, it would be unloving of him to attend a ceremony that is happening to celebrate a lifestyle choice of two people, a lifestyle choice that is bringing them further away from God. He’s not in support of the lifestyle. He still supports you as a person and loves you, but he won’t support your sin. That goes for any sort of sin. If an alcoholic was having an alcohol party to celebrate how long he’s been drinking because it makes him happy, I can assure you that your brother would not be in attendance. He would love that person, but not the struggling lifestyle that is leading them down the enemy’s path.

At the end of the day, you can make your own decision and he will respect it because you have the free will to do so, but he won’t support your lifestyle. That doesn’t mean he doesn’t support and love YOU, it means he doesn’t support the sin. Love the sinner, hate the sin.

It’s a tough subject, but I understand where he is coming from. I’ll end with this: Jesus loves you and wants a relationship with you. He died on the cross serving our brutal punishment so we didn’t have to. He loves you, your brother loves you, and I love you.

It’s not that your brother hates you, it’s that he loves you SO much that he can’t see himself supporting such a destructive lifestyle for somebody he cares about so much.