We arrive in Rome May 2 and have booked a tour for the Sistine Chapel and the Basillica. I believe it all will be closed. Want advice if we should cancel tour
Since you don't know it is a fact that it will be closed it makes more sense to wait until the tour company that you booked the tour with to tell you first that your tour has been cancelled and give you details about your refund.
While safe to say that the Sistine will be off limits on the 2nd, I don't know as that applies to the Basilica nor the larger parts of the Vatican Museums as there's nothing on the websites for either stating that fact. Still, as advised above, you need to contact the company you booked with for further information.