Robyn I'm so sorry for you. It's so hard to be an animal lover who cares for and takes very good care of her animals like you, to watch an injustice being done to a poor innocent kitty. I have animal idiots in my family too and it breaks my heart when I see or hear certain things.
I think you should certainly have a talk with your cousin if for no other reason than to know that you tried to prevent anything like this ever happening again in the future. There really is nothing more that you can do. I would want that peace in my heart anyway. I tried, it might not work, but I know I tried.

Prayers for poor Tutter.