I Think My Cat Is Sick....Help Me Please!!?

I got 2 kittens sometime in June or July and when it grew up a bit I started to let them go outside. One kitten was gone for about three days and when I went out to find her she was inside the undreground of my house and came out. She seemed weak and had problems walking. I checked the back of her and her "private" was very pink and open and had a tiny bit of leakage and her sister is hissing at her and growling and running away from her. Can someone tell me whats wrong with my cat. Is she having babies or really sick? I don't have enough money for vet bills so I hope she isn't sick.