Would you let your son to play fight

My SIL has a two year old son and whenever he sees my baby he touches him in a gentle way and then he wants to pinch him. Ofc I don't allow it. My SIL then told him that he can play like that when my baby is older. I didn't say anything (thinking she was kidding)  but when my baby is older and he hits my son I'm going to the over protective momma bear and tell him not to. My husbands family got raised differently than me- the type of family that curses in Spanish. And sadly I live with them and my MIL will be taking care of him when I to Bak to work. 

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