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This is a tough situation, but the reality is interfering with the other parents choices is not only unfair to them but it is also unfair to your child. While you can refuse the gifts, the fact is someday your daughter is going to resent you for interfering in her family relationships.
While they may be trying to "buy" her affections the reality of the situation is if you are raising your daughter to be a strong independent women then no amount of gifts are going to change who she really is.
Someday she will be mature enough to see this for what it is and then she will make her feelings known, but to step in and take away her family's rights to gift her is not going to go over well with her or her family. Not to even mention the fact that this would not look great in court should they ever attempt to fight for more custody or visitation. These kinds of actions can be tagged as family alienation.
In the end her mental well being is far more important then anything else and denying her the right to gifts purchased for her is going to cause more harm then good.