Bad attitude

We have been visiting my mother in law and my step father in law for 2 weeks since it’s summer and we live 4 hours away.

My niece who is 7 almost 8 used to live with them till she was 6 (sister in law is a single mom and financially needed the help) welly son is 1 almost 2 and her daughter is so rude. They don’t discipline her at all and she used to be the only grand child till she was 4 but there are now 5 of them. And she literally cries over everything

My son was trying to play with her in the play room and as I’m sitting there she goes and cries to my mother in law that my son pushed her. Which he never even touched her and then my son get yelled at. He wants to play he doesn’t have siblings yet and isn’t in day care because my parents watch him while I go to college. I love my sister and mother in law. But I’m fed up that my niece keeps crying and saying my son did something he never did.

I discipline my son with timeout or we have a talk. He is one so he is still learning but she literally cries about everything and keeps going are you leaving yet.

I don’t want a family fight but if this continues tonight on are last night I am temped to tell my husband that we won’t be coming for Christmas because my son is becoming so timid because he keeps get yelled at.

Any ideas on what to do. My sister in law doesn’t take criticism or anything bad being said about her daughter. But I’m about to say my son can’t be around your daughter because he is picking up my nieces bad attitude.

We’re ttc #2 and this is just stressing me out and I have not slept much for 3 days and I’m ovulating and I’m afraid this situation will mess with me getting pregnant.