My partners daughter is 10 yrs old, we have lived together for a year now. We went through/still are to some extent going through some of the things you are going through. DH daughter went to see a child psychologist earlier in the year due to a lot of the issues. She got herself to the point that she couldn't fall asleep at our house and would site stomach aches etc. She also had issues with me and at times my daughter who is also 10, and her mother interferes a lot, demands to know every detail of what goes on at our house and it leads to many conflicts. According to the psychologist there are a few issues at play, first is jealousy. She wants her dad to her self, to be the centre of his attention (she has 2 brothers, but she's daddy's little girl). The psychologist said the jealousy is normal and to expect it, and it will manifest itself in many ways including how they act towards their dads partner. The other thing she mentioned is that they need to have time with just their dad with only bio siblings because they still need that sense of belonging. I too struggle with this issue tremendously and at times it seems like an impossible feat especially because of how bio-mom feels about me and how she verbalizes that to the kids. Hope this lets you know that your not alone.