And a bit of a germaphobe.
I have a wacko SIL who I avoid as much as possible. Me and most of the men in the extended family and many of the women. Not manly, but she's easily offended, so it promotes harmony.
However, I do at times consider yanking her chain and seeing if I could shock her into silence. In the case of your problem girl, I'd probably start talking about wanting to hang glide, do an extreme run, possibly take up mud wrestling (you have to post pictures if you actually do that one) and anything else I could think of that might be so beyond her frame of reference it would make her leary of me. I'd line up a set of these, and when she started to complain about one say something like "yeah your right, hang gliding isn't back to nature enough for me, what would really be cool is to go on this survival hike where the trail is marked with bear scat..." Do keep a little distane. It'll be messy when her head finally explodes.