One option would be for your MIL to gat a FHA refinance with you as a non-occupying co-borrower. You might even get a better rate to help out. Another option would be to buy the home from your MIL as your second home, pay off the old mortgage and let your MIL pay you whatever rent she could afford as long as she lives there.
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Ron is a mortgage Loan Officer specializing in FHA and USDA loans but in 2008 due to all the owner financed notes on the market, began a new company that buys private mortgage notes. His note buying site is
http://www.sell-my-note.biz