Estate Law Questions? Ask an Estate Lawyer.
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If you are challenging the will and validity of the changes, you will bear the burden of proving that your mother was incompetent mentally at the time she made the changes. This also means you will need an attorney, because the law will require you to be represented to file a will contest. You will need to file an objection to probate and will have to show the will is not valid as it was improperly changed and this is the only way you will resolve this matter.
The sister exerting her influence over your mom based on her close relationship with them is called "undue influence" and it is grounds to invalidate the will. Only a court can do this and you have to file the objection to will in the probate court.