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As a practical matter I would take the position that all the equipment you were the signer on was a loan and that you want the loan paid off in full. Failing that you will take the equipment. Shut down the business and open it under another name with just you and your wife as owners. Tell you customers that you are leaving the business and leave it up to them as to where they want to go.
This approach is on the legal edge but I doubt that your brother in law or the mother have the wherewithal to fight it.
It is very difficult to have a person declared incompetent. That would be a very time consuming process and even if successful, you would have to deal with the person appointed to govern her affairs. If you think she is not competent, you should retain an attorney as we only provide general information and not legal advice.
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