I think you have to accept that you are not the mother of your husbands children from a previous relationship. The 12 year old does not accept you in that role. The term "step-mother" doesn't really mean all that much. I mean: it can mean a lot, if the child is open to that, but this youngster is not. Leave the "parenting" to the child's father. It's his responsibility. If he is totally failing in that, then you are in a bad spot that will probably get worse. It's already so bad that you are thinking of ending the marriage. That may be your best bet. Then you can raise your one year old in a peaceful environment, without the tensions of a child there who is disruptive.

It's not surprising that your 12 year old step-daughter has all these problems, given the background she is coming from. I'm amazed your mother is even willing to put up with all this. This is a real tough situation to sort out. There isn't any good option, just a choice between hard options. And it's probably going to just get tougher. Definitely try some counseling, if you can. But things are still going to be tough.