What Victoria says is quite true: consult a real estate attorney. Also, the standard MLS contract used in the Capital Region provides stiff penalties for early termination including compensation of expenses, loss of commission, damages, etc. So you have to be careful.
However, we at Real Estate New York http:RENY.net always cross out the cancellation clause so that a client can cancel at any time without penalty, We just wouldn't want to force someone to stay with us if they are unhappy. It's a recipe for a tough sale at best. Haven't had a cancellation in years.
Hopefully, the agent your sister signed up with feels the same.
Good luck : )
You should really speak to your attorney about discrepancies with your contract, but this is really dependent on who the executor of the estate is. If you and your sister both have equal power to sign, I would imagine your attorney should be able to get you out. If one of you has executor power, it will depend on who.
All the best to you.