You are in a really difficult position, and I can see that.
This isn't the same thing, but I can relate a little. I had a very dear friend who identified herself as pagan. We'd already had some conversations about Christianity (funny I was not REALLY a follower of Christ then, but stood up for the idea) and because of that, I already had some positive influence and an "in". THEN I became a true Christian. Then she invited me to a gathering which was going to be largely pagan. (No rituals or anything like that, btw.) I felt terrible, and like I should not attend, or if I went and "preached" they would all reject me. If I attended I would be "approving" it. If I didn't, she would be offended and I'd lose my friend and any possibility for influence. Well ... It took a lot of prayer to decide what to do.
I'm going to say this. If you feel strongly as though you'd be denying your faith and approving homosexuality for going - you must not go. I think the principle in Romans 14 applies:
14 I know and am convinced by the Lord Jesus that there is nothing unclean of itself; but to him who considers anything to be unclean, to him it is unclean.
If you yourself see it as sin to attend, then you must not.
However, I want to say that I see everyone else's point. Many times evangelists may purposely visit areas populated by prostitutes, drug dealers, and any other manner of sinners, and basically meet the people where they are. That's what I believe Jesus would do. I don't seek out those kinds of evangelistic activities, but because of non-Christian friends, I have been thrown in the midst of gatherings of drug addicts, homosexuals, and yes, pagans (I did decide to go to the gathering above). And more than once I have been ASKED to talk about what I believe, and why. Faced with a room full (dozens at times) of people very antagonistic to my faith, I'm going to admit, it was a little scary. But the amazing thing is, I think it was arranged by God and I knew how to answer and have given at least some of them something to think about.
My point is, I understand the other posters urging you to go.
Right now in this country, homosexuality is a "special" battleground. I don't know why. But it is. You've been put in a hard place, and as I said, as long as you feel it's wrong, you shouldn't go. But sin is sin, and unbelievers of all kinds need the love and forgiveness of God, just as all believers needed it.
I hope at some point you are able to share your love and your faith with your brother. In the meantime, you can pray for him. Perhaps God will send someone else, or perhaps He will make you able and make the opportunity to use you.
I have prayed fervently for 12 years for my brother to be saved. We've been having "seeker conversations" for years. I love him dearly. But in the end, it was someone new he met that helped open him to the faith, and I praise God someone did!
Peace to you, and I will pray for you and your brother.