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If you are back-up POA, then you would only have authority to act if the primary was incapacitated and unable to act, was removed by your parents, or resigned. Otherwise, you wouldn't have any legal authority to step in and act on their behalf.
If that is the case, then your recourse would be to petition the court where they are located for guardianship over them. There is no other way other than going to court if sister-in-law does not agree to resign. That would give you full legal power over them and their assets and revoke any other POAs. You would need a probate attorney in TX to assist with filing the petition.
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