In a carboy, with an airlock in another house.
Seriously, if your MIL uses the space for crushing grapes, you don't want to contaminate the area on the off chance of infecting one of her wines. Most of us just do beer, sanitize everything and boil all our wort. The risk of cross contamination that way is minimal, and even then, a lot of us will use separate hoses and other equipment that is hard to guarantee is sanitized.
In a carboy with an airlock and you should be OK. What temperature is the wine cellar? It could be too cold for year-around storage. You probably want it to get to the mid 60s or 70 for at least part of the year.