Wow. I wouldn't say, as in the previous answer, that this child has severe anxiety who is hitting. There certainly may be more behind it, but generally, when verbal skills are lacking somewhat, kids tend to act out behaviorally. In other words they may be thinking "Hey, she has a toy I want and I really really want it." But can't express that adequately so figure if they hit the other kid, they'll get that they want the toy. It's not always malicious. I always try to ask myself "If this was a stranger's child, or someone's child who is not in my family, how would I handle this?". I would definitely have a gentle conversation with your SIL. Just let her know that you would prefer if she steps in when she sees her child hitting yours. I love the advice of Laura U. who advises you say gentle words to each child, and give positive attention towards your daughter who is being very patient. I agree sleepovers may be too much at this young age. Try to keep the visits short and if there are toys that the child tends to fight over, or things they find difficult to share, then just try to remove them prior to visits. In the end, the child is only 3.5, but even at 3.5 children can understand the behaviors their parents find unacceptable. You really need to enlist the help of your SIL to intervene. Good Luck!