Would you do everything possible if. . .?

My stepson who lives with us and has for awhile, has began saying he wanted to move back with his mom when his mom finally started calling him after she realized the judge made it clear she does and will have to help support her child, which she hasn't done. Well, she went from only calling him after my stepson would try and contact her for weeks, and she finally would call back, to calling him EVERYDAY. Well, when he said he would move back she bought a ticket for him to fly out to her. She does not have a stable lifestyle and just isn't in the position to put her sons needs above her wants, so no way would we send him to her. Well, when my hubby made it clear to my stepson he isn't going anywhere he put up a half hearted short fight. Afterwards we thought he would be totally pissed, but he was relaxed and friendly, more then he has been in awhile, and no it didn't seem like acting, as he would have just went to his room instead of spend the rest of the evening and night with the family. Anyway, now we have her sister (all family lives close to us,) interdering. Some things we heard her say, etc. sounds as if she is going to be taking him to the airport if we do not, with or without our permission. We contact our local PD and they explained she cannot do this as she is neither a parent or legal guardian and that it would be kidnapping without my husbands permission as he has lived with us for over 6 months (he has lived with us for over a year.) How do we explain to her without it sounding like a threat that if needed we will do this, that as a teacher she does not want to get invloved (she could lose her license to teach,) and needs to let the PARENTS deal with issues with their CHILD.