There’s a pretty comprehensive article on this question on
this site. It’s written by Michelle Arnold who is quite orthodox. I would say that her prescriptions are pretty hard line and most Catholics I know would attend more ceremonies than she allows.
So, even though your brother and his wife aren’t Catholics, because of previous marriage, there are impediments to the marriage. That makes these two points most relevant to you:
The Church does not explicitly forbid Catholics from attending presumptively invalid marriages. Catholics must use their own prudential judgment in making the decision, keeping in mind the necessity to uphold the Catholic understanding of the sanctity of marriage. To make such a judgment, you might ask yourself if you believe the couple is doing the best that they can to act honorably and according to the truth that they have. For example, you might decide to attend the presumptively invalid wedding of a couple who is expecting a child (thereby attempting to provide a family for that child); but you might decline to attend the presumptively invalid wedding of a couple you know to have engaged in adultery (thereby destroying previous marriages and families).
While there may be just reason to attend a particular wedding that will be presumptively invalid, I cannot recommend participating as a member of the wedding party in such weddings. There is a difference between attending as a non-participating guest and actively involving yourself in the wedding.
I think you can clearly see that they are doing their best according to their own truths and so you have no reason not to attend. Michelle Arnold suggests you may have reason to decline to participate in the wedding party. That part is up to you.
Note that the Church does not give hard line prohibitions here. She says she cannot advise it, but you are not forbidden from doing it. It is a matter for your own conscious.
My advice (admittedly as a non-Catholic) would be this: if you love your brother and you can make the day about him, do it. Your relationship with him will last for the rest of your lives and making him feel you reject him doesn’t do anything to help either of your or bring him closer to God. If anything, it will drive him away.
However, you know yourself better than any of us. If you agree to be in his party, but are going to be unhappy the whole time, making occasional remarks about how this marriage isn’t valid, that they should be talking to priest, whatever, then tell him up front that while you love him, you can’t do this, decline to be the wedding party, and go as a guest. He deserves to have a wedding party that supports him completely.