You haven't specified the particulars of your mother-in-laws “messiness”. Is she getting over a bad situation? Hopped up on something? Incontinent? There’s varying degrees of help required in each situation, but the best solution is probably the one of least interference. Be the big man and allow this walking complaint to shack up with you, but after that, just let the woman run wild and free.

Alternatively, you could put her to work for you, what I like to call “earning the bed”. Start off with some small tasks that seem like idle requests, like “Could you find the remote and hand it to me?” or “Could you wash this dish?” Gradually increase those requests until you’ve got (another) live-in maid.

If you don’t feel like exploiting this person who you have only a superficial familial connection to, exploit your girlfriend and her more-than-limited sense of tolerance for the woman that raised her. Create annoyances and blame them on her mother, as you would a small child or pet. “Who left these muddy footprints in the hallway?” “Must have been your Ma, the prints are clearly orthopedic slippers.” And you’ll have the place to yourself again in no time.