Step dad vs step daughter

[Disabled]

Please help. I've been with my partner for 5 years and lived together for 4. We both have children from previous relationships, I have one he has more but only sees one. My daughter lives with us full time his treats the place like a hotel. Because my daughter is with us FT he thinks she should have more responsibility. They get on all the time apart from when his daughter comes to stay, she stirs things up and he is too blind to see this. She then causes friction and goes home as if nothing has happened.
My daughter on the other hand is a moody teenager and suffers from depression due to domestic violence from her father who we no longer have any contact with.
It's getting me really down as on the whole of things my daughter and her step dad do love each other but when his daughter comes to stay the love turns to hate. Any suggestions?