3 Year Old Toddler - Bad influences

Hello Moms!

Wondering if anyone can share some advise.

My husband and I lost our jobs and had to move into my MIL home. We’ve been here over a month now and my son is picking up all these bad habits from his 2 year old cousin.

For example, when my soon goes potty in his diaper... he tells us and lays on his back right away... his cousin kicks and screams and runs away to put on a diaper. Well my son now does this. He kicks and screams and it’s exhausting.

My son used to share fine with all his other friends and cousins but... with his 2 yr cousin they are always constantly fighting over the same toy and don’t like to share with each other.

Our son listens when we tell him only smell grandma’s backyard flowers but when the other one rips them off then my soon follows and he gets in trouble because he’s a big kid and cousin is only a baby.

I’m so tired. I know these things will happen anywhere school, friends, etc... and I try to use the big kid vs baby thing. Tell him big kids get to color and read fun construction site books... but then my SIL will bring her 2 yr old and want her son to be a big boy. I feel like I’m confusing my son even more.

Any tips would be greatly appreciated!