If your roomates want you out of the apartment, they must bring a proceeding against you in Housing Crt. (Holdover or Non Payment). Changing the locks is an illegal eviction, that makes them liable for treble damages.
http://tenant.net/Rights/illevict/illevict.html
You may want to go to Housing Crt on Monday and file an Order to Show Cause and tell a judge that you have been locked out of your home by your roomates. Ask the Clerk or Pro Se atty what the procedure is...Call the utilities companies and get an account print out to prove your residency there...
You may also want to call the police to escort you back in to the premises. Your rommates must provide you with a copy of the key.
Regardless, of if you have given a vacate date to them, or haven't paid rent etc... changing the locks is illegal unless they have a court order (warrant to do so) prior to you vacating. The LL has very little say in this matter as you rented from the sisters not here, and he can't prevent your access either.
If you can handle it live out the remainder of your month in the apartment. Why should you be further inconvenienced?!?