Toddler Hitting

My niece is bad about hitting with toys and I'm not sure what if anything I can do about it.

She's just turning 2 and my son is 3 months older than her. Every time we get together she ends up hitting him in the head with something. I think its a product of him trying to take a toy and her not wanting to.

We are working with our son on taking turns and listening when someone says they don't want to, but hes only 2. I just feel so badly that he's always the one who gets hit in the head with the toy. He doesn't currently hit and I don't want him thinking it's an okay thing to do.

Her mom is there every time, and reprimands my niece but not until after it happens. I comfort my son and tell him that his cousin doesn't understand sharing but I don't know what else I can do.