Caught in the middle

I have an open adoption, and that means my family has an open adoption too. A little while ago, there was a minor issue that got blown way, way out of proportion between a family member and the adoptive parents. The Adoptive father is very protective of me and wanted to make sure I was okay with everything going on in their relationship with this family member. Since then, this person has not been a fan of him, and I feel caught in the middle. I want to maintain my relationship with this family member, but I won't tolerate anything bad being said about the adoptive dad because he was trying to protect me and I appreciate so much what he did. How can I ask this person to just leave it alone? I've tried and no wording has been successful