Could my son be bullying his sisters because he’s being bullied by his father?

So my ex husband has 14% custody and the kids go over to his house every other weekend. But when he’s at his dads his dad takes his kids to his parents Saturday and Sunday, which it’s supposed to be his time with them but always just had his parents watch them, anyways him and his father are very narcissistic and manipulative which is part of why we’re divorced in the first place. When they have the kids they treat my daughter like she’s a little princess, the grandparents always wanted a daughter but never had one so they do a weird favoritism with girls, so they are very sweet to her but very ruff with my son. The grandfather tends to “bully” him more, he’s very ruff and very physical, he doesn’t beat him but he rough houses a lot with my son and he pulls ears, and is just a physically manipulative man, he does the same with adults as well and has made me very uncomfortable. My sons father is nicer to him but can be verbally abusive.

When my son (who is 4) comes home from a weekend there, especially when they have been at their grandparents my son becomes very aggressive, he lashes out more, doesn’t listen to me when I ask him to stop and becomes very physically aggressive and bullies his sister more than normal. During the week with me he is very kind to his sister, they of course fight occasionally but he doesn’t bully her to the same extent as when they come home from the grandparents. I’m worried the it could be that he’s being so badly bullied by his grandpa that he has bully someone else to feel in control of himself. Aside from correcting him when they are with me I’m not really sure what else to do, I don’t even know how to approach it with them because I’ve asked their Father Time and time again to stand up for his kids and keep his father in line when they are over there. On the flip side how do I document this in case it becomes a bigger problem and I need to protect my children from them? I currently have the majority say and final decision legally, but since we’re all getting along fairly well right now I’m afraid to make issues.

So I’m looking for other parents opinions or from someone whos had to deal with something similar, does it sound like my son might be doing this to his sister because of how they treat him or could it be something else? Also how would you document it and keep track of it for possible court reasons down the road?