Actually, I did say that the mom is non-caring (and non- supporting)! And I stand behind that. What you have told your children about their bio parents is lovely, and probably makes them feel wonderful.
But imagine how this boy would feel if he were told the truth - that his "mom" has the legal right to see him, but doesn't. And that she is supposed to give his dad money for him, but she doesn't. This poor child could get serious feelings of worthlessness from that. (I hope he doesn't know).
She hasn't made some noble decision to let his father have him - the COURTS decided that dad was better. They gave her the right to basically prove herself with visitation and support, and she didn't. And six years later she decides to try and pull this poor kid out of his home, after the trauma he has suffered by losing his baby sister? That does not sound like a caring, warm, loving mom to me.