my bikeless sibling in WI asks " the big question: If all my time in the saddle from now til March is spin classes of 1 to 1.5 hours, will that be enough training for 100 km on March 25?"

It could be, especially following what Venus de Velo said.

I'd suggest paying attention to diet, on spin days what do you eat before and after. On long rides keeping fed and hydrated is important.