Can one attend the wedding or wedding reception of a Novus Ordo relative?

Thanks for your question.  We did address this question in more detail in Question 62 of the Questions and Answers section, so we refer the readers there.  The short answer to your question is absolutely not.  A Catholic cannot attend the wedding or the wedding reception of a heretic.  The reason is that to attend such a wedding service or the reception is to honor and celebrate the marriage of a heretic.  It is to honor and celebrate a person or people getting married in the state of mortal sin, and in a fashion which displeases God and places them on the road to hell. This issue involves the divine law:

2 Corinthians 6:14- “Bear not the yoke with unbelievers: For what participation hath justice with injustice?  Or what fellowship hath light with darkness… or what part hath the faithful with the unbeliever?” Pope Pius XI, Mortalium Animos #9, Jan. 6, 1928: “Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment ‘Love one another,’ altogether forbade any intercourse with those who professed a mutilated and corrupt form of Christ’s teaching: ‘If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you.’ (II John 10).”

This means that Catholics cannot have any interaction with heretics or unbelievers which gives them the impression that you accept them as good people or respect their false religion.  Attending the wedding or wedding reception of heretics certainly gives them the impression that you accept them or their activity as good.  Precisely, it is to honor their heretical activity.  But Catholic Faith obliges you to hold that their marriage as heretics or in a heretical fashion is something that is displeasing to God (even if it is a valid marriage).  The only truly charitable approach is to calmly explain to your brother why you cannot attend his wedding or reception, emphasizing the necessity of his conversion to the traditional Catholic Faith.