Two sister cats terrorizing my roommates cat

Question
I have two orange tabby cats that are sisters.  They are five years old.  My roommate and I moved in together in August and she has a long hair female calico who is just over a year old.  The three cats were tolerating each other for the first four months, althought they never got along.  Now my two sister cats are literally terrorizing my roommate's cat.  I found a great deal of blood on the floor yesterday and tufts of fur on the carpet.  My two are keeping my roommates long-hair cat from using the litter box and are pretty much ganging up on her and ambushing her regularly.  I dont know why this is happening all of a sudden, or why its getting worse.  What can i do to pacify the situation?  I feel horrible for my roommate's cat as she is stuck under the bed most of the time now.  I've never known my two cats to be so aggressive and preditor-like.  Help!  This is stressing my roommate out horribly, as well as myself!

Answer
Jen,

All of the cats involved need to see a vet to rule out medical reasons for their behavior. In the meantime keep the separate so that these attacks are prevented. If you found "a great deal of blood" on the floor that is an extremely serious fight and you probably need the vet to look into anti anxiety drugs for all and a referral to a behaviorist to stop these cats from killing each other. Due to the severity of the issues that  you have presented me with I am not prepared to give advice beyond seeing a vet, explaining the situation and looking at referrals to a behaviorist and medications to treat the aggression and anxiety. I hope that I managed to address your concerns with this answer. I do apologise for not being more specific, but as I said this is an extremely serious case of aggression and should be dealt with by professionals so that everything is done right the first time. If you have any further questions or concerns, please do not hesitate to contact me again. I will do my best to give as accurate and timely advice as possible. Best of luck with this serious problem.