My 12 yr. old step son is sneaky and manuipulative and his mother caused him pain. how do i deal w/ it?

i married a man who did not live with his children and visited with them every other weekend. His 12 year old son lived in Georgia with his mother. Last year he came to live with us which was only about 6 months after we got married! He is sneaky and has told me that he does not have to listen to me and lies often. My husband has a lot of guilty feelings over his son being in GA w/ his mother as she was never a good mother to him. He is acting out because of his mother and is taking it out on me instead of being angry at his mother for the things she did to him. He is disrespectful and unappreciative to me and at times my husband however at times it's like in my husbands eyes he can do no wrong as he feels bad for him. Since he has lived with us our marriage has not been good. Anytime we tried to do somethig together w/o my step son he causes some type of problem to cause my husband and I to not be able to go out or we do go out and my husband is angry and or upset w/ what my step son had done at the time. He has told me that he does not like me and wishes his parents were together. The thing is he was not a planned baby and my husband and the other of my step son were never married and were not together any longer while my step son was still very young. And my step son tends to complain tht something is also wrong with him on a daily basis such as his arm hurts or his stomach hurts or his leg is sore, etc. I miss my husband and I have a hard time talking to him in my own home because my step son is always around. It's very frustrating! My 18 yr. old step daughter is wonderful though! I am only 30 and do not have children of my own. Any suggestions as divorce has already crossed my mind several times because of my step son. Please help!