Total Life Change But Loving It

I met my husband online and it has been the greatest relationship ever in my life. He has two wonderful child, daughter 2.5 and a son 7.5. These children and amazing and i love them like my own. My stepson lives wihhis grandparents in the next state so we dont see him much which sucks. His daughter we have abot 4 days out of the week. This little girl is so smart and beautiful i love her very much. She knows her abc and countd to ten in english and spanish. The only problem that i have is with her mother. She says i should be greatful she isnt a witch but in all reality she is. She makes my husband upset and tells him that hes bad and stupid all the time. She is ruining my life because she upsets my husband to the point where he struggles and it affects his work. I will remain with him forever but she is making my life crap because my husband is upset. Everyday it is something new with her. She must be boted in her life to try to control his and i cant stand to see him upset. The only one that is gonna suffer is the little girl and i hope she doesnt turn out like her. Im hoping in gaining some advice of how to deal with her biological mom. I love these children but his daughters mom is ruining his life.