If your bf likes pics of his ex’s family would you be mad?

He has his ex’s sister on Instagram. The sister posted a pic of herself and my bf’s ex and he liked it. He never likes any pics of the sister by herself or anything else she posts, but he did like the one with his ex in it. It honestly really upset me and I don’t know if I’m just insecure and crazy or what. I understand his ex and him have a history and he knows her sister and all but I’m just bothered by it. If I bring it up to him I know it will probably sound crazy. How would you feel? Should I just let it go?