My Husbands ex wife expects so much of me what should I do?

Right, I really don't know how to word this all so I am just going to blab it all out as it enters my head! My husband had a girlfriend in the past he was 21 she was 16 they were together for 2 months and he decided to call it quits, she announces she was pregnant so he thinks ok he wants to do the right thing so he marries her! Baby is born and not even 1 year down the line he has had enough and divorces her. She has custody of the child and has the child during the week and my husband on weekends - this works fine (We were all in Africa at this stage) he then met me we are together for 2 years then marry and have a daughter and move to the UK. She follows us here and even moves into a house 3 roads down from us. She then gives us the child and says that she will have him on weekends because she needs to work and get her life sorted out, I agree because I stay at home with my daughter and my husband is happy he has both his kids with him. Fine, but the thing is, it has been 8 years now and her son who is now 11 wants to live with her. She has a partner and her retired father living with her now too. She just wont have it. My stepson is becoming almost imposible to have around and has started to steel things from us like money (not large amounts but still) he wants to live with her, he lies all the time and constantly tries to hurt my other two children. He brakes their toys deliberately and hids them away etc etc. I have always treated him as my own and made sure he has always had everything he needs. Some weekends she tries to make him stay with us when he does not want to and he performs about this. She is extremely petty about silly things like cheap socks that she has bought him that have landed up at my house that she wants back (even though he has gone to her house with socks on in the first place, who cares which pairs are at who's house!) She then takes this out on my stepson and shouts at him for not bringing back the socks, or whatever it may be at the time. Despite all this all he wants is to be with her and does not want to live with us anymore. She phones us constantly when he is at her house and wont listen to her and she never calls during the week to see how he is unless there is an issue that I have brought to her attention. Anyway this all leaves me feeling terrible - I hate myself for it but I am starting to resent having him in my home, and I feel that I am paying the price of having a child at 16 when I never had a child so young so why do I have to deal with all these issues all the time. I love it when he is with his mother and it is just me, my kids and my husband. How do I cope with all this, how do I tell my husband this, and what can I do? I am at my wits end with all this. I have even considered taking my children and leaving but then I dont know what would would happen to my stepson and my husband works horrible hours and cannot take him to school and I know she wont do it and I will be made out to be the horrible person in all this! Help!!!!