Wedding of a Non-Religious Brother

My brother is not religious (comes from an ultra-Orthodox family) and is marrying a non-religious girl. He is considering taking a rabbi from the Tzohar organization (a rabbi authorized by the Chief Rabbinate). Is it obligatory to attend the wedding to maintain family ties in the hope that this will cause him to return (apart from minimal meetings at events, I have no connection with him)? Alternatively, is there a problem with attending? What is the criterion at the wedding that should stop me from attending, i.e., what halachic prohibition that will occur there should cause me to leave the wedding?

Answer of our teacher, the Gaon, Rabbi Amram Fried, shlita:

You may attend the wedding.

And leave if there are mixed dances.