Under SSI, Is it a feasable plan that when my sister recovers from two forms of cancer that she sells the house to pay me back the money she owes me.

If your have a loan agreement and a promissory note with a mortgage recorded in the county deed records, your loan will be repaid out of the proceeds of the sale.  If your sister then has no more than $2,000,she will continue to qualify for SSI.  Since she may have more, she may want to consult with a local elder and special needs lawyer about putting the house (and thus, later, the money from its sale) in a special needs trust.  The viability of this approach varies partly by county.  She can find a local attorney who specializes in this area through the Special Needs Alliance or by using the Find a Lawyer function on the website of the National Academy of Elder and Special Needs Law Attorneys (www.naela.org).