Through most of your post, I was thinking about how I'd normally think/say something like how you're not dating her, you don't need to trust her, you only need to trust your husband. Express your concerns to him and set boundaries about not allowing her drama into your relationship, and let him handle his own relationships. But then you said:
Originally Posted by Fiona
and I don't feel like I can trust her (or my husband) either.
Though you put "or my husband" in parentheses, as though it is an aside, it's a big deal. It's not a trifle. It's important. And telling. And scary. This needs to be resolved, and fast.
What do you not trust, and why? What needs to happen for him to gain (or regain) your trust?
In general, for my own relationships, I just trust my partners and let them deal with their significant others; there's no reason for me to need to trust the significant others BECAUSE I have trust in my partner's capability to manage relationships.
I'd also be especially concerned about how/why this girl's drama has spilled into your relationship with your husband, and with your other partner. This should be unnecessary. Can you give examples of how this has happened, and what impact it's had?