Estate Law Questions? Ask an Estate Lawyer.
It is legal as long as this person is not assaulting your sister in law. If the ultimate goal is to protect your sister-in law and her assets, consider obtaining an adult conservatorship giving you rights over her body and a complete estate plan including a trust, will, durable power of attorney and healthcare directive giving you rights over her health decisions and assets. That is the only way your sister-in-law could be fully protected. Having a local estate planning attorney would be most valuable under these circumstances to ensure legality.
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Saying "it will most likely not stand up in court" does not conclusively establish it will not stand up in court. Perhaps having a no contest clause in the will and just executing the documents with a qualified local estate attorney is your best alternative. Otherwise, you can leave the circumstances as is.