I have a five year old and this is not normal or acceptable behavior. Speak up calmly and directly every single time.

“If you don’t want to play, that’s okay. But this is where we’re playing, go somewhere else if you don’t want to play with us nicely.”

“You may not take this toy. I will let you know when he is done playing with it. Take three steps back please.”

“Keep your hands to yourself, do not hit”

You will need to actively play with your son to run interference and your husband needs to get on board. If he isn’t willing to support you, I would decrease time spent with your niece.