Fiance's daughter

Ok so my fiancé's daughter keeps beating up and bullying my two kids. I have tried to do everything to get her to stop. I have talked to him, and he repremands her but it continues so even though it broke my heart because I love this little girl, I asked him to move out until they both get some professional help. I'm just at such a loss on how to help them. But since I have asked him to move out my kids are so much happier. My son has stopped throwing tantrums and my daughter's confidence has gone through the roof.