Hi, and welcome to the forum.
I can tell from your letter that your step-son's relationship is causing stress, but it's unclear as to why?
At 18 and 17, they should be mature enough to handle a relationship...and yes, if he's over 18 you really don't have control, except where it comes to who is welcome in your own home.
You seem to be targeting her, may I ask if she is truly causing trouble? Or, is it just that you and her husband just don't like her, her morals or personality.
I'm a firm believer in not tolerating evil. No one will come into my home and my family if they are destructive. If she's the type of person that lies, gossips, or deliberately undermines peoples relationships, then I wholeheartedly agree with you that she isn't to be allowed in the home.
But, if you just don't like the way their relationship is going, this "immaturity" you speak of, then it's probably better if you try to just be polite.
There are several possibilities here: Either they'll break up after a while and she'll be out of your life...or they might marry or have a child together. If that's the case, she'll be in your life one way or another for a long time. Either way, it will be in your and your husband's best interests to learn to tolerate her.
But, that's only if we're talking personality differences or just that they are an annoying couple. If she really is destructive or evil...stick to your guns that while he's free to do what he wishes outside, you and your husband can and will draw the line at the front porch.