A wedding reception is not just a family party/reunion. It is an event that has a particular purpose: to honor and celebrate a wedding that has just occurred. This particular wedding is occurring because your relative chose to apostatize from the Catholic Church so that she could marry a Muslim. In other words, she chose to entirely abandon the Christian faith because she placed her feelings for a non-Christian above her relationship with and commitment to her Lord and Savior, Jesus Christ. That is a legitimate consideration you must keep in mind when trying to decide whether to attend this wedding reception, not a well-meant but misplaced idea that reciprocation for kindness means that one must attend the wedding reception of that person’s son.
Another legitimate consideration is that even though this bride has apostatized, that act does not free her from the obligation to marry according to Catholic marital law. (It used to be considered the case that a formal act of defection freed someone once a Catholic from marrying according to Catholic marital law, but the Church in recent years has determined that it no longer does. See the document
Omnium in Mentum, linked below, for more information.) For her marriage to be presumptively valid, the bride must obtain permission from her local ordinary to marry a non-Christian in a non-Christian ritual. If she does not, the marriage will be presumptively invalid.
You say that the bride “knows of [your] dissent.” In some cases less serious than this, that might be sufficient. But in this case, this bride has committed some very grave actions that have the possibility of endangering her very soul. How will she understand the gravity of those actions in time to be reconciled to God and the Church if those who love and care for her protect her from experiencing any consequences for those actions in the here and now? If her family all shows up for her wedding reception as if this marriage were something to be celebrated, she will have no reason to reconsider her actions. If you conscientiously choose not to attend, perhaps she will.
Keep in mind though that this is your decision. I can only give you information to consider. Ultimately only you can decide whether or not to attend.
Recommended reading:
What are the rules for attending weddings?
Formal Defection by Jimmy Akin
Omnium in Mentum by Pope Benedict XVI
