That's not cool. It would be important to know whether your mother is still alive, and, if not, whether she was alive at the time your sister added the requirement. If so, then it would be good to know (i) if she had power of attorney; (ii) how broad were those powers; and (iii) when the power of attorney became effective (upon signing or incapacity). Remember, though, POA's expire upon death.
It probably wouldn't hurt hiring an attorney.
The above statements are provided as general information and not intended as legal advice. Each matter has its own set of unique circumstances that cannot be adequately addressed without consultation. You are strongly advised to hire an attorney licensed to practice law in your state to represent you.
Your sister has no authority to modify or augment your mother's will. If your mother is competent, she can modify, adjust or change her will, as she desires, so long as the will conforms to the legal requirements for will execution in your state. If you need more assistance try the Avvo Find a Lawyer tool for a local attorney.
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Your sister doesn't get to change the will to suit her fancy. It sounds like you may have a will contest on your hands. Talk to an attorney immediately. If you know where the original will is located, take it with you to the attorney. You don't want it to go missing. We offer free consultations. Remember, you have time limits for probating the will. It is best to have title in your name sooner than later.
This post is for discussion purposes only and is not intended as legal advice. This post does not create an attorney-client relationship. If you are interested in retaining counsel, you may contact The Law Office of Daniel O'Brien, P.C. at 512-615-3580 to discuss further.