What you describe is not binding--not because it is in email (binding contracts can be created by email), but rather because it does not meet the criteria to be a contract. A contract requires "consideration," or something of value, from each side to the other side. She was not receiving anything of value for her promise to waive her right and let the house be sold. Without her receiving consideration, this not a contract but only a non-contractual, or as it is often called, "gratuitious," promise, and a non-contractual promise is not binding; it may be freely reneged upon.