“This is tricky. Legally, the home belongs to the daughter.”
I agree. Depending upon the exact circumstances and the laws of your state, the daughter may or may not have the ability to enter into a contract where she would sell the house to you or anyone else. But again, she may.
It probably depends most critically upon whether the mother has been declared a legal conservator for the daughter. If she is a conservator, then the daughter can sign nothing. And it also depends upon whether there are other heirs who might contest whatever you come up with. It also depends upon how disabled the daughter is. If the mother has NOT been legally established as conservator, then who knows the daughter is disabled? Does the daughter have an actual will or is she (and everyone else involved) depending upon intestate succession?
I am not a lawyer, but my brother is and I have worked closely with him on several estates, including our parents, for which I was the administrator (A term used when the estate is in the form of a trust)
Let me work sort of backwards. The situation you want to avoid with the greatest revulsion imaginable is that where a decedent (dead person) dies intestate (without a will) or, with an improperly drawn trust.
The only way it gets worse is if the the decedent is non compos mentis. (hint) You have not stated the nature of the daughters disability. Mental? Physical? Both?
The ideal situation you want that will guarantee the most predictable outcome is one in which the assets of the decedent are held in a properly drawn trust. This is better than a simple will, because most small estates bypass formal probate BUT WILLS ARE MADE PUBLIC; which means that should the mother owe some creditors money, they could attach or lien certain assets in the brief period of time between when the daughter dies and the home passes into the mothers ownership. If the house is the biggest part of the daughters estate and the mom owes somebody $50K, the house could be liened or otherwise attached while mom owns it. And that claim would predate and thus trump yours, even if you came up with some sort of cooperative plan with the mom, even if you paid the late taxes. You probably have not talked to the mother as to whether she has any debts outstanding and she may not disclose nor remember them.
My reco would be to find a skilled estate lawyer. You may have to 1: pay the atty to create a trust for the purpose of guiding the chain of ownership of the house, 2: pay the tax arrearage, and 3: pay mom some kind of option consideration. By bringing up those ideas, I do not intend to short circuit what a real estate atty (AND ABSOLUTELY NOT JUST A “GENERAL” atty, a REAL, SPECIFIC estate attorney) might say/do.