Hello everyone I live with my husband's parents and have a son who is …

Hello everyone I live with my husband's parents and have a son who is 2.8 years old and a daughter who is 8 months old. My son hit my daughter. I have repeatedly explained to him that it's not allowed to hit his sister but when he hits her I can immediately reprimand him. Just now, my niece scolded me because my son hit her and I reprimanded him. She says: "I lived worse than you, I didn't even get to live. My father was bad and my husband why are you so nervous? You live as you want and your husband is good." Every day I have to clean up and prepare for my niece when she goes to work