It would be best to talk with your landlord rather the rent dispute committee. what's your concern is definitely in the contract which binds your sister to a legal obligation. it was you who lost the job but your sister still has.... in which also the flat contract was named after her. I bet your sister has released cheques until end of this year. The only thing you and your sister can do is to talk to your landlord and ask if he may reduced it to one month only. That is a win win solution, you are out of the flat and the landlord still has one month to post the vacated flat. You dont have much choice now, your landlord has your cheques.