Sorry to say, but there are not many options, as others have stated. It is possible to get a B-2 nonimmigrant visa for obtaining medical treatment. I think a medical treatment B-2 visa holder can stay much longer than a normal B-2 visa holder. But it is not an immigrant visa, and your mother would have to eventually leave. Plus you would have to show that she cannot get the necessary treatment in her country, have proof that a doctor or medical facility in the US is going to treat her, and show that your mother (or you) have enough funds to pay for the entire visit plus medical treatments.
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