I'm also a stepmom to a 17-year old boy. You have my sympathies, I am also experiencing problems getting along with my stepson and it is very hard to try to please everybody.
Soensie - right up front let me advise you to please, not let your husband take the easy way out here. You need to sit down with him and have a good, long discussion together about this problem. This problem is your husband's problem as well - she is his daughter and he is the custodial parent and guardian, so he is reposnible for her upkeep, routine, disciplining etc. Telling you that either the daughter goes back to the mother or you split up, is a cop out - it's the easy way out for him isn't it? It's also very unfair because it leaves YOU stuck in the middle - I know, I have been in the same situation and it's a b*tch.
I suggest you take the time to be on your own somewhere quiet and calm, with paper and pen, and make a list of everything you try and help this girl with, that means cooking, cleaning, housework - everything. List all the rules of the house; list everything that you and your husband agreed to do TOGETHER for the family. Make a note of what you like about your stepdaughter, and what you don't like. Note down her behavioiur as well and what you have done to try and resolve the problems, if they haven't been resolved, put them down. Once you have done this, only then try and arrange for the two of you to sit somewhere where you know you will have plenty of time to talk together. Don't point fingers and accuse and say "YOU won't let me" or "You are unfair.." that's guaranteed to upset him and get him on the defensive, which you don't want to do. The calmer and more rational you are, the better. men don't respond kindly to hysterics and tears, been there done that! Try to persuade him to look at the situation from YOUR perspective; appeal to his sense of fair play and right and wrong, but do it as neutrally and as calmly as possible. Tell him you have always been a willing stepmom, you want to help your stepdaughter to be the best she can be, but you can't continue trying to do this when she behaves in an irrational manner - I would tell him that her attitude and behaviour is causing the greatest friction and upset in the household and putting a strain on your relationship, which you do not want to happen at all, and you would like him to help you resolve the problems you are experiencing, so that your home can get back to normal. I would definintely (tactfully) tell him that his treating each daughter unfairly is not on, and is causing problems which need to be sorted out as soon as possible. You love him very much and you always want to be together - tell him that too! You should also point out that it it having a detrimental effect on your daughter as well, and you do not feel this is a suitable environment for either of the girls. Appeal to his emotions too in this way - a picture of constant screaming, tantrums and strife, or all of you cohabiting in peace, calm and domestic bliss.
I don't know how you feel about my suggestions. I have tried this method of resolving problems in my house, and in some cases I have succeeded but in others I have failed miserably, to such an extent that I have given up being a stepmom and taking an active part in the stepson's life. It's extremely difficult to handle and it's not going to go away overnight. It's also very hard to try and stay calm and unemotional - I learned the hard way that losing my temper was fatal, I would immediatley lose all credibiliity once I lost my temper or got so upset that I would burst into tears from sheer emotion and say the wrong things!
I hope for your sake, Soensie, that you manage to resolve the problems you are having with your stepdaughter. Us women always have to work harder then men in a relationship and make more sacrifices - fact. If you reallly love your hubby and are prepared to stick it out, then I say, go for it. It may take a long time to fix this and only if you stick to your plan of action, which is the most difficult thing to do - and in the end, if the situation is still the same or worse, then it is time to step back and ask yourself - where to next? What is the next step?
Be strong, girl :-)