The home should have a title similar to a vehicle title. Either it is paid for and in her name, or there is an outstanding loan and the lender holds title until it is paid off. She will need to make provisions for its distribution in the event of her death, either by will or otherwise. It is unlikely you will be subject to capital gains unless the mobile home is worth a whole lot of money and I do not believe capital gains applies to mobile homes. Check with a lawyer who handles estate plans/probate for more detailed information.
It would be wise for your MIL to be proactive and provide for this distribution in her will, assuming that is her intent.
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It sounds like your mother built a home on your property. It APPEARS that you and your husband are the owners of the land and, thus, with the home being an improvement on the land you are now the owners of that.
With that said, if your husband has brothers or sisters, then mom should draft a basic Will to keep the others happy (or less unhappy).
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