Don't assume it's her mom manipulating her, because if may not be true. Do you live close to her? If you don't she may be worried about seeing friends which are important for teens. She may just be more comfortable at her moms house too since she's a girl and all. Maybe try small steps, asking her if she would want to go out to dinner or shopping then give her the option to spend the night at your house or go to herr moms. If you force the issue it won't help your relationship.