Fostering nephew who's hitting my children

Hello all, I'm hoping someone can help me as me and my family are having a very difficult time. Back in Nov 2019 I recieved news that my nephew was taken into protective custody and was told by my sister that my husband and I were the only viable placement options. Now, I don't get along with my sister and have avoided contact with her for the past 10 years. I believe she is a sociopath and I avoid my mother as well. With that said, I agreed to go through the process to care for my nephew whom I barely knew, and who will be 4 in March. I have two children of my own who are 3 and 1. From day one, my nephew was violently hitting us, but my DS has taken the brunt of it. I'm talking welts, nots on his head,scratches that bled and being violently shoved. My nephew is older and substantially bigger then both my kids and the aggression has now reached over to my 1 year old. Today we caught him stepping on the baby's hand and watching how his face reacted. It wasn't until my husband called over to him that he finally stopped. My poor baby had a shoe mark on his red little hand. It was creepy and concerning the way he did that. It is to the point where I'm afraid to leave either of my kids alone with him. It's not a question of if he will hit, but how bad or when. I want what is best for him, I really do. But at what point do we end this placement with the safety of my own children in mind? Has anyone experienced having to have a child removed from your home? I'm torn, but feel my duty to my own children and their safety comes first. Either my husband or I have to be present at all times when they're all playing because he will hit. Even at the grocery store he hits and pins my son against the cart by his neck. He's not all bad and can be cute and loving but it's hard getting past the violent behavior towards my smaller children. I don't see the behavior changing soon. We have worked with him for two months and he actually seems to be regressing.