One rat biting the other, drawing blood and scabbing

Question
I have had these two rats for a couple months now, got them both at the same time, I believe they even may be litter mates. Both female, both seem to get along very well. They roll around and fight from time to time, but nothing serious. Lately I've noticed a large scab on the throat of one of the rats. I didn't know what it was at first, but then I caught the other rat pinning the injured one down and chewing her neck. The hurt one squeaks a little, but otherwise lays there and takes it. I just separated them and blood came away on my hand. I'm really worried and not sure what to do about this. Thank you for the help.

Answer
I'm leaning toward the idea of it being overgrooming/violent grooming rather than just flat out aggression. The submissive one - the one who's hurt - is submitting to her sister/cagemate rather easily if you haven't heard any massive fighting - and usually rats who are getting aggressively chewed on won't do that. Instead, it seems as if her sister has just gone a little overboard on grooming/loving and is actually causing more harm than good, despite her intentions.

The alternative is whether or not the submissive one hurt herself prior and now her sister is trying to "take care" of the scab, or if she actually caused the scab. You could separate them for a time to give the injured girl a chance to heal up and relax, then re-introduce them and see how they behave together - or you may need to separate them permanently. You could also introduce another female (if you have the room) and distribute the level of attention from just one rat onto two - provided she isn't openly aggressive to a new roommate.

Good luck!