It is unlikely - but this is best handled through your attorney that knows the specifics of your case and this sibling bickering that has had numerous post here.
She is most likely stuck with the agreement; although lawyers are paid to find holes in agreements, as you know. If she really tries to break the agreement, it will put the attorney for the three of you into a conflict that would be too great for that attorney to represent her (at least regarding the attack on the agreement). You would all have to get independent counsel, probably. I don't think your sister will go that far or waste the money she would otherwise get by paying it to an attorney to attack an agreement that will probably end up being valid and binding. Good luck.
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