Preteen relatives hurting my kids

This has been happening for YEARS. My husband’s nieces are 12 years old. Ever since my son (now 5) was born, they’ve been abusive towards him - plugging his nose when he was a baby so he couldn’t breathe, shoving their fingers so far down his throat he gags, and as he got older hitting, kicking, knocking him down, and most recently slamming his head on a concrete floor. We avoid them like the plague, but this is not possible at family gatherings. He is afraid of them, and I tell him to just stay away from them. It’s to the point where I don’t allow him to go to my MIL’s bc she lies and says they aren’t there when they are. What do I do? How do I handle family events with them around? I have another child who is terrified of them and wants me to hold him anytime they’re around so that saves him but what else can I do? This is a tragedy waiting to happen. My husband and in-laws pretend to not see. When I tell nieces to stop it, I get dirty looks from the rest of the family. They are in trouble in school for this behavior so it’s not just my kids experiencing it. Do I avoid all family gatherings?