I'm going to call my sister stupid for having another kid
Why do you feel she is dumb?
She spoils her kids a lot and I can't stand them, they are always running around my house
It is wrong to judge family for having kids if they want them. You can just not invite them to your home if they can't behave.
I just think she should stop having more kids because they are not my responsibility.
If she wants to have kids it is her body, her choice. You didn't marry your sister did you? So there is no reason you would have to take care of the kids anyways.