I am sorry this is happening to you. I would be glad to help.
A functional marriage is like a picture of two folded hands. It is hard to tell where one begins and the other ends. In this case, there is a huge block to that joining in the form of a step daughter...one who is acting far outside of her normal given rights and status.
First of all, you are totally correct here. Any child, mature or still at home, cannot be permitted between the married couple. The couple, no one else, must be the priority in the marriage or the marriage will fall apart. But here your husband is allowing his daughter to act with what is called, equivalence. That is, she is being given the right to permit herself into your role, and to threaten, upset and violate your emotional security, the marriage integrity and your wellbeing. She is also showing signs of jealousy and control, an issue that she should be addressing in a functional way, not through threats and immaturity.
You are so correct. If she is not stopped the gap between you and your husband will increase and increase.
What should you do?
Even if he will not, I would suggest that you see a counselor. Like iron sharpens iron you need emotional support from an unbiased professional. Also, with you attending counseling it opens a door for him to go, far more than if it was initiated with a requirement that counseling involve you both immediately.
Couples counseling is the most demanding and difficult type to do well for a professional. I teach professional counseling and of the 20 students we graduate each year, only a few can do couples work.
Therefore, if your husband was completely unable to move in counseling before I would not give up hope. It may have been him, but often it is a combined issue that also involves talent of the individual therapist.
The best way to find a very good couples counselor is to ask your PCP, or if they do not know a good resource I often encourage people to call a local university that has a psychology-counseling program. The department head almost without fail will know the best of the best in your area and can make a good referral.
It is not easy to change a man who may feel (typical emotions that drive this lack of support ) guilt or shame. But it can be done, and I would strongly suggest that you seek out someone who can support you face to face. I have seen men, even like your own husband, change. However, often it takes a realization that the relationship might end or that this is indeed very serious, but you are worth the effort to try with him. Crisis often brings men to awareness. It is unfortunate, but often that is what it takes to have issues of this magnitude resolved. But you are hurting and need support through this. And, you are right. She is out of line and is harming your marriage and your husband has an obligation to support you first, not her. Even if he does not see it, you do, and that must be enough for a man to support his wife above all others. Steven