I'll try to keep this brief, there's sooo much going on. For background my partner and his wife divorced 9 yrs ago and I came along 3 yrs ago, the first time I met the children they all confided disturbing antidotes from their life with their mom.

In general I'm constantly on edge about what their mother is going to do to us next, She's constantly making wild allegations against my partner and even myself in public spaces. I am worried about what she's telling the children, They have gotten pets and rewards for bad behavior and not coming to visitation.

So here's one of the most troubling issues currently. The oldest, who is now 15, was distant at first but after a few days with dad became happy and vibrant and we got along well. At the end of dads two months she was inconsolable at the thought of going back to her mother, same thing happened the next year. She begged him to move closer and we were ready and able to so we did. 
After about 2 weeks with her mother the oldest child started to become hostile to her father and me. 2 months later she physically attacked her father for grounding her over her behavior and we have not seen her since. 6 months ago he spoke to her on the phone, trying to inform the children of a death in the family, and she cussed him out and accused him of abuse and trying to get her mom in trouble. Recently the 10 yr old has started becoming hostile as well.
The legal situation is ongoing but very favorable that the children may be placed with us permanently soon and we have a psychologist we want to take them to.

I hate to say it but I'm terrified of how the children will react and how to cope myself.