There are already a lot of good replies here, but in response to the OP, I would say that sure, it is normal for kids to be somewhat difficult in relating to others - particularly younger relatives in their home environment...they can sometimes feel very threatened (I have a little experience with this when visiting my brother´s family - last time DS was 2 1/2 and DN was 4). However, being repeatedly physically violent to another human being should never be acceptable, regardless of the circumstances and no matter how much one might understand where it´s coming from. You certainly do not need to tolerate your son being pushed into a wall. If you are not in a position to take a major part in trying to help DN improve his interaction skills with DS (which is your choice), then it is not overreacting to limit the time they spend together...especially if you are 7 months pregnant - I would think you have a enough going on.
I don´t have professional advice or specific info. to offer, but I think the best way to handle thes types of situations is to be there in defense of your DS and try to thwart off any aggressive behavior before it happens - and just remove DS if it becomes too much. This seems to go far beyond the "try to let them work it out" theory...I know many PPs have already said something similar, but ultimately, you need to do what is best for you and your son. If he has plenty of other playmates with whom he can spend a pleasant time - even if it involves a little pushing and grabbing here and there - which is normal - I would try to focus on that.
It must be difficult because your mother is involved, but hopefully you can find a way to communicate your position gently, without too much negative reverb.
Good luck with this...
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