I think so, too. I do not know of a priest or bishop who would forbid it. If we are not forbidden by conscience or circumstance from attending a sibling’s wedding, we are obliged.
It would make it much easier to talk to one’s pastor for guidance, though, if one had misgivings, if for no other reason than being assured that it was OK. It would make it much easier to enjoy and add joy to the celebration.
**OP: **If your parents are suffering over this, too, make an appointment, go with your parents, take the letter in question, in case it is asked for, and talk to your parent’s pastor about this. If they’ve accepted the situation, then go by yourself to your own pastor. This is a situation where some pastoral help to discern and accept the situation might make a difference of night and day.
If your priest sees all the facts and has a chance to question you and consider your entire situation and still forbids you to go, then don’t go. Apologize for any hurt feelings, be very sure to let him know how you regret the situation and still want his every happiness, if you didn’t do so already when you let him know the first time that you told him you wouldn’t be attending. Bend over backwards to make this about his situation with the Church that you are still a member of, and not about him or what a wonderful and I am sure God-loving human being your future sister-in-law is.
Otherwise, go, and with your priest’s pastoral blessings, I’m sure.
And take this consolation with you, too: Remember that to your brother, this is about pleasing God instead of pleasing the Church. While he is in the wrong, he is in the wrong for the right reason. He is (at least in his mind) not doing this to please himself. That makes all the difference in the world. When you talk to him about religion in any way whatsoever, keep that most important desire that you do both still share always in the forefront. Even if it were to become clear that he does want to please himself, and is using God as a pretext, treat that pretense as if it were his real reason…because if you do, it is more likely to become his real reason. His desire to stick to the religion he was raised with, if that is what this turns out to be about, will be blunted, if the religion respond in the recognition that God is the most important thing in the conversation. That is the bridge between you, and between him and the Church. Concentrate on that.