There is an obvious equitable interest in the property when one pays taxes on the property for over thirty years. Also, if your mother lives on the property and has done so for those 30 years, there is certainly good reason to believe she owns the property. It would be helpful to know how your mother originally acquired the property. Also, more details are needed to understand the concerns you raise by mentioning your sister's warranty gift deed and survivorship. I would definitely have your mother contact a real estate attorney with more details if I were you, just so you can understand a little better what might be happening.
Your mother absolutely needs to hire a real estate attorney who practices in the area where this property is located to research the title and find out what, if any, claims she may have. I don't know what a "Warranty Gift Deed and Survivorship" is. If it is a Warranty Deed, it must be from someone who held legal title to the property and one question would be whether that person held title superior to your mother's claim of title (if any). Only a real estate lawyer who has reviewed all the documents surrounding this can properly advise of your mother's situation.
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