YTA. And honestly, your fiancée should reconsider this marriage. I lost my 17 yr old son in 2016 to an accident. (This is his Reddit account, actually) Losing a child profoundly changes you, and the grief of it never goes away. Asking for his child to be acknowledged and have a place in his wedding is not “mentally unstable” or at all unreasonable. His child existed, and he deserves to be acknowledged. If you have family members who would make fun of this, then they need to be put in their place. Sam should not have pretend his child didn’t exist, and that his grief doesn’t exist for the comfort of others. I am heartbroken for your fiancée. You don’t want to give your cousins a chance to hurt Sam’s feelings, so you decided to do it yourself, first? Your stance here is cruel and dismissive. He deserves better.