E.,
I had the same sort of issue with my nephew. He likes to push and plays real rough. I think it's due to the fact that all his cousins are older and they watch wrestling and play real hard with him. Both my son and my nephew are around 2 now.
When I first started noticing my nephew taking toys (we teach our son to share, but taking toys someone is playing with is not OK.) and pushing and stuff, His parents didn't say anything. They seem to think it's funny that the younger cousin is doing this to the older (by 2 months cousin)I didn't voice it but talked to my husband about it.
My son wrestles on the floor with dad and likes to have tickle fights with people, so I'm ok with a little "rough house" play, but not take it away, I want it, push you out of the way you could get hurt type of stuff. We are not chummy with them either.
We finally decided if his parents wouldn't say anything we would. So next time he pushed, I told my nephew no...and I still do this. They haven't really said anything to us about it, but I've noticed, when I tell their son not to do something they start paying more attention to how he's acting at that time.
I guess I would just take up telling your neices and nephews that's not OK, not yelling or anything. If the parents have a problem, then that's thier deal and then they can take the initiative to talk to you about it.
You should be able to voice your opinion about how your kids are being treated, even if it is just to the kids.