I totally agree that you should not attend his wedding. That would be best.
If he and his wife invite you for dinner or Easter or 4th of July or Thanksgiving or Christmas or New Years you likewise shouldn’t attend. You wouldn’t want them to think that you approve of them living in a non-sacramental, non-Catholic marriage.
If they have a child you shouldn’t celebrate the birth or the christening. That would be best. You certainly would not want to look like you approve of their non-Catholic, non-sacramental union having produced a child, and their raising their child in some non-Catholic non-denominational church.
If your brother is ever hospitalized, you should not visit him. Unless he is hospitalized in a Catholic hospital, visiting him would be condoning a hospital that wasn’t Catholic and therefore probably performs sterilizations and abortions.
If he ever ends up in jail for any reason, you should also not visit him. That would be best. You certainly would not want to look like you condoned whatever behavior caused him to have ended up in jail; it would be a terrible reflection on you and your family.
If he gets hit by a car and dies, you shouldn’t go to his funeral. That would be best. His grieving widow is not going to give him a proper Catholic burial. You certainly wouldn’t want to condone his dying without repenting, without last rites, and without a proper Catholic funeral, for gosh sakes.
You say that you love your brother, but your brother has turned his back on the Catholic Church and so what he is doing is wrong. You yourself would never turn your back on the Catholic Church, of course not… nor would you ever bother actually reading nor obeying the Catechism of the Catholic Church which states very specifically:
(818) “All who have been justified by faith in baptism are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers in the Lord by the children of the Catholic Church.”
I’m proud of your brother for showing you the love, honor, and respect of having invited you to share his joy on his wedding day but I totally agree with you that you should not attend any wedding if all you can share with your brother is your own ignorance of your own religion.