I think you have a good grasp of what is going on. The daughter enjoys being around you and her father but feels guilty that she is in some way betraying her mother when she expresses this. Mother obviously feels competitive with you all and is also resentful. Here are my questions/suggestions:
- Do they have an official custody arrangement that spells out visitation/custody? If so, she should not be planning activities during her ex’s time.
- The pre-teen is old enough that her dad can speak to her about it being ok to enjoy time at both parents houses.
- Don’t put her on the spot when you call her at mom’s house and ask her if she enjoyed her weekend. She has a great deal of loyalty to mom (and dad) and doesn’t want to hurt anyone’s feelings.
- Consider counseling. It sounds as though she feels very conflicted about the divorce and how to relate to her parents.
- Is it worth pursuing more custody or modifying custody?
Hopefully she will realize more when she gets older but it is important to communicate now. It is normal for daughters to side with mom but mom should not be attempting to alienate her from dad. This is very serious.