It just depends. In Michigan, this *could* be upheld. But it would be considered suspect because the ordinary formalities were not observed. Who is going to testify as to your mother's intent and capacity? Not the attorney, because there was none. Not the witnesses, because there were none. Your sister's testimony would be suspect. It is also possible that a court could find that she exerted undue influence on your dad. Of course, worst case scenario for your sister is likely that the codicil would not hold up and you simply go back to the original Will.
You probably should review BOTH documents with an attorney to see what effect the codicil would have and how best you would challenge it. Medical records showing diminished capacity would also be helpful to your cause.
Obviously, this is going to affect your relationship with your sister, one way or the other. This is certainly something to think about as you are determining how best to proceed.
Best of luck to you!
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Generally codicils have to have the same formality as wills. In addition, it sounds like this is a situation of duress, overreaching and fraud. You need to meet with an estates litigation attorney immediately to discuss your options.
Hope this helps.
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