Unbelieving brother remarrying. How to respond?

Greetings. I have an unbelieving family member, my brother, who divorced recently. From everything we know, I would say it seems to be a case of adultery and abandonment, with him at fault. He is now engaged to another woman and soon getting re-married. We, as Christian family members, are not sure how to respond to this.

My believing parents have sought counsel from their elders and received conflicting answers. Their pastor has said that we have no business attending the wedding, regardless of the risk of damaging the relationship between us and him, because being there would be supporting his wrong and adulterous decisions and we need to be firm on how we believe God has designed marriage to be. Is this sound, biblical counsel? If so, how far does this extend in our relationship with him beyond a wedding?

Other counsel has advised to be clear with him that we don't support the divorce and remarriage, but go ahead and attend the wedding and support him there because he’s family and we love him and we want to keep a good relationship between him and us.

Thoughts or advice on how to lovingly and faithfully approach this situation?