You don’t need a clever remark in a situation like this. You just need a firm “Hey. Let’s not do this right now. Let’s have a nice meal together.” The *family member* says this, to their family. Then the family member takes it offline with their family and figures it out. Later. Never should the S/O be sitting there fighting it out with the family or making it a “Me / Her” thing. At the end of the day, this is a sibling thing and they need to work it out with each other and their parents.