fuming- in laws not following our rules!

There is clear research on babies and children and screens and the harms of it. We've explained to my in laws. I left a camera on when they offered to watch the baby and they had my baby in front of the tv the entire time! My baby is only 4 months old. They propped her in front of the damn tv for about three hours straight! 
My husband of course doesn't want to intervene. What do I do?