You are correct, if your father does not have the mental capacity to know the nature and quality of his actions, then he cannot do another will.
You can’t do one under a Power of Attorney either.
Your system was adopted by your father, and she wants a claim on his estate as a beneficiary she is going to have to come up with some proof that she was adopted.
There would be a certificate of adoption which is a very valuable document very similar to a birth certificate. If she has lost it, she can get a copy but nonetheless, if she is going to maintain that, she’s going to have to produce the document.
If the house is in half your name and half of your “adopted” sisters name, it doesn’t matter whether she has been legally adopted or not in respect of the house but it may do in respect of any other bequests to daughters under the terms of his will. If she has been adopted, she is treated as a natural child.
Can I clarify anything else for you?
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