My cat seems to be sick or something really strange, please help me out.?

My poor cat Dorie is sick or something, I'm not even sure. My family and I are very confused with what is going on with her. The fur on her backside hasn't completely fallen out, so when I search for it online I don't know what to look for. It's almost like its, shaven, because the hair is there, but very thinned and short. Its only around the base of her tail and her back hinge legs. She use to be the dominant female in the house, among her own brother and sister. But over the past few months she really hasn't been looking well. I don't come from a wealthy family and we really do live paycheck to paycheck. If this is something serious then we will need to save money to take her to the vet. Not to mention Dorie's brother Fee Fee has 3 legs. Long story short, that cost about $600, and that was a few years ago, when they were only little kittylums.