Stay calm -- it sounds as if you are pretty scared right now. Keep your son's best interests at heart. A child's biological parents are almost always terrific influences and the ones a child wants to be with. Yes, I did read that your son cries when it's time to go. My daughter has cried for years. We are very open & talk about this with her. She'll say that her dad & stepmom are mean and she doesn't want to go. (I do think she's been mistreated, however, I would not let her go if I thought she was abused.) I've talked to her dad and stepmom, telling them that they are ruining their relationship with her -- they need to really hear her and work some things out.
Does it hurt to still send her? Absolutely -- it's crushing. But I believe a relationship with her father is important. And when she says they are mean, we discuss the fact that she gets to see how they choose to live and she sees how my husband & I choose to live. And when she gets older, she'll have both family lives to guide her in the proper decisions in her life. Does she want to be mean to others? Or does she want to communicate and love others -- as unconditionally as human beings can.
Please keep your son's best interests in mind. It WILL be tough. Stand up for his rights and what is reasonable for him (keeping in mind school, homework, sleep, etc.). If another woman wants to help love your son, embrace that. You will always be his mother & he'll always love you more. Don't feel pushed aside by her! Be fair and strong when you go to court, and make sure you have a lawyer that agrees with your morals/decisions.