Toddler being too rough

Moms I need help!

My 3.5 year old has always been really gentle until recently. We started seeing his boy cousins again (who are very rough with each other). I’m guessing he learned this from them since he was never like this before.

He started getting really rough with his 1 year old sister. Tonight was pretty much our breaking point. I know he didn’t do it on purpose and didn’t know he could hurt his sister by doing this. We’re just stuck on how to explain to him to be more gentle. Not to touch people unless it’s with nice hands (hugs or sharing). He shoved a toy in her face and caused her to bleed.

I really want to nip this in the bud before he gets worse and before he starts school this fall.

Any advice is welcomed! ♥️