Sister rats not getting along

I have two female rats that are two years old. One is a hairless rat and the other has hair. Last night before I went to bed the rat with hair was pining down her hairless sister and had her laying on her side. I rescued these two girls and have had them for about a month now. They have never acted this way before. I do believe that they both have mites also as they are always itching and scratching. What is the best way to treat the mites and is this behavior normal in rats?

I have seen it in some of my rats. It doesn't seem to last or turn into anything real violent, it almost seems like a show of dominance or who's in charge.