My cat is sitting weird, tail up, and her butt is wet. What's wrong?

Poor Mini Marm! Given her hind end discomfort, wetness, and interest from your other cat, my main concern would be that Mini Marm is having problems with her anal glands. These are normal scent glands that typically empty with a bowel movement, but can become impacted, infected, or can rupture. Other possible causes include bite wounds, injury to her tail base or pelvis, among others. I strongly recommend taking her to the vet for evaluation, because it is quite likely that antibiotics and pain medication may be needed. Do not give her any over the counter human pain medicine, as none are safe for cats. Until you can get to the vet, put an e-collar (cone) on her to prevent further licking of the area, as this could make matters worse. You can also try using a warm compress on her hind end for 10 minutes 3-4 times per day if she will tolerate it. I hope this helps and that Mini Marm feels better soon!