train is likely easiest way to get from Siena to Pisa
Ciao David!
Sundays tend to have limited schedules but you definitely can get from Siena to Pisa, either by train or bus.
By train you need to travel on the regional trains from Siena to Empoli, change train in Empoli and then head to Pisa. The whole trip takes around an hour and 50 minutes. Not sure of the cost of the trip, but shouldn't be more than 10 euros. Go to
http://www.ferroviedellostato.it/homepage_en.html to search for the time schedules.
The Siena tourism office recently also started offering a shuttle bus from Siena to the Pisa airport. One way ticket is 14 euro for the trip and also takes about 2 hours. Schedule is more limited, with daily departures only twice a day at 7.10 and 13.20. It seems to run on Sundays, since there is nothing that says it doesn't. You can get more info and make reservations once you're in Siena by calling the toll free number 800-913-231.
A third option is to take a bus from Siena to Florence and from there take the train to Pisa (trains every half hour). For the bus schedule to Florence, check out the second page here:
http://www.sitabus.it/sita-toscana/Firenze - Siena.pdf - only the times with an "H" above run on Sundays. You can buy tickets at the places indicated on the first page.
The train seems like a better option overall but check the bus schedule too to see which one works best for you.