I have two wonderful nieces one maybe a step niece but I don't care she is a niece. My step nieces father is dead and her mother is cheating on my brother. They are now getting a divorce but truth be told neither of them deserve the children. He is although loving and giving to both children is sometimes absent minded. She on the other hand is showing the girls that it is OK to abuse your husbands both mentally and physically. She is an abusive mother as well mentally anyway and isn't mental abuse as bad as physical abuse? My question is Should I fight for custody of the girls? Could I win? And what would it take?

Here are a few things about me to help with possible answers.
I have been married for 6 years I have two children and my husband lets me be a stay at home mother while he works for the U.S. Army. We do live really far from them right now. We are ready to give them the things they need and are willing to still let them visit the parents for the summers, and winter breaks from school. HELP PLEASE!

First he/you would have to prove she is an unfit mother and I doubt any amount of verbal abuse short of serious threats at the children count as abuse enough to have them taken away from her. Your brother can file for primary custody and he can try to get that.
If the one girl is not his and it is proven he isn't then it is a possibility he would only get his daughter and not the other.
Unless your brother can prove she is unfit and wants them signed over to you then there is nothing you can do.