I would consult an estates, wills attorney in either FL or MA to cover all your bases, but absent some action like setting up a trust I do not believe this will be possible.
Your wife cannot attach money that she does not have unless her mother permits it. But this would merely be a gurantee then. That is, the loan secured by your mother-in-law assets, which of course is possilbe.
Remember too that things do not always work out like we plan. There is always a chance that this estate has some issues and they may frustrate your objectives.
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