Yes, it is relatively easy to take the train from the airport in Milan to Milano Centrale, then on to Florence's SMN train station. From there you can continue by train to Colle Val d'Elsa: the town itself is divided into two parts, as the historical part is on a hill and the newer part is below and the train station is here in this part.
Is your hotel in the newer part? The only negative thing is that the train connection from Florence to Colle is not direct, you go through Empoli and you
might have to change train there (not always required).
Another drawback is this: do you really want to subject your 88 year old MIL to 4 different trains?
How about treating yourselves to a private transfer from Milan's airport all the way to Colle, with maybe a stop along the way to take a break during the drive? It might even cost less, than the 4 train tickets for two.
Here are just two possibilities - I'd suggest emailing both and asking for a more precise quote, giving them your dates to ask for availability.
http://www.absoluteitaly.it/tours_milan.htm
http://www.italychauffeurservice.com/leisure-travel-services/city-to-city-transfer.html
What do you think of this as an alternative? Otherwise, train is doable, just will be a long day!
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