Oh dear do I understand what you are going trough right now!

I have this 4YO part timer that is always on my son's back... hitting him, pushing him, making fun of him... my little guy is turning 18 months this month, just started walking, getting some molars in and is very sensitive to things like that...

I told her mom that she has 1 week to work with me on this behavious because it is NOT acceptable. MY son is usually a really happy baby... and he spent all day yesterday crying and we are on the same patern today...

I would have a very good talk with the parents explaining that divorce is not an excuse for awefull behaviour! I think your sister will understand