needing advice!

I have an 11 month old son, he has a 2 and a half year old cousin. My sister in law and I both do gentle parenting. My sons cousin is SO mean to him. I know toddlers have no impulse control but I just don’t know what to do. He has punced him in the face MULTIPLE times, if my son pulls up on anything (not even remotely close to his cousin) he will purposely kick him in the chest so he falls off, he tackles him and takes ANY toy out of his hand and screams in his face and more recently I had to get my SIL because she had a migraine and couldn’t see to drive, my nephew was pinching my son the entire car ride, making him scream hysterically and leaving marks. Im always in arm’s reach and luckily and able to intervene 90% of the time, some times my nephew is just too quick.

Today they met us at storytime at the library. My son and I go every single week, he has so many little friends there. Everyone (20 kids) were playing on the parachute and my nephew had no issues with the other kids playing but the second my son was having fun he full on pushed him to the ground and kicked him.

My sister in law doesn’t do much to correct him other than pull him away for a second and ask if he needs a hug. I don’t want to over step but I also don’t want my son to get seriously hurt because of this. My sister in law is very emotional and I know will get mad if I don’t address is super sensitively. Should I try or should I have my husband talk to his brother?