She has a two year old son and three step children. She despises the step kids. When her child was born she wanted their father to take a month’s “break” from the other children to bond with theirs. He told her he couldn’t take a break from being a parent and would take care of all his kids.
She throws a tantrum any time he does anything with his other children or if they come over. She calls the seven year old fat, and has banned her from having snacks. Even though the two year old can have them. She has gotten so bad that she even makes up lies on them to try to win people over to “her side”. Like telling people the teenager is inappropriate with the younger children. Nobody but her has ever witnessed it.
I have tried to reason with her. She just gets mad at me and accuses me of being judgmental and keeps repeating that her son shouldn’t have to do without a father because his ex drops those “brats” on them constantly. It’s gotten to the point to where I am missing out on my nephews’ life because I can’t be around that.
LemonVibe 
Then take a step back if you need to. The father knows and is who should deal with her. Stand up for the kids when you can. Call child services if she physically harms the kids or gets scary. Call police immediately if you see her harm the kids. The kids aren’t blind to the fact she hates them and the damage between the father and them is dine. The teenager will start to find ways not to visit and so will the others as they come of age. By then he will have decided he has had enough and moved on from your sister. 50% of first marriages end in divorce. 70% of second marriages. 90 % of relationships after first marriages with kids because of stress coming along with kids.