Unfortunately, when you marry someone, you marry their family. My husband could bore you to tears with what a pain in the rear end his in-laws (my family) are.
What I really recommend, though, is that you step back and look at the situation slightly differently. Sure, you'd like for this young woman to just go someplace else so you can get your regular life back, but really, she sounds like she could use your help. What does she want to do with her life? What career would she like to pursue long-term? Great, she'd like to X? Well here's a community college catalogue, and it looks like they have classes that'll get her the first step of the way. Oh, and she's really concerned about Y problem in the world? Well, they really, really need volunteers at such and such a place.
In other words, rather than simply pawning this 26-year-old "problem" off on someone else, help her take a few practical steps toward building a life, a career, and a friendship network. That'll get her out of everyone's hair and onto her own two feet.