Lease agreements usually hold that all signers - residents and co-signers alike - are held jointly and severally liable.
That means that if your sister flakes and leaves, the roommate AND you are EACH responsible for ALL of the rent. Ditto if the roommate flakes.
The landlord may not be amendable to what you suggest. You should contact the landlord and ask if such an arrangement can be made. Don't be surprised if he says no, though.

