Hi, Timon! Glad you're going back to Orlando. That means we can all be treated to an update on your terrific site!
I've done the connection through TO to Orlando (and other US cities) several times. It's pretty quick, but your sister needs to be really clear about where she picks her luggage up before proceeding to customs. . . it will not be checked through; she has to retrieve it and carry it through herself, before depositing it back on a conveyor on the 'other' side. The AC agents at the arrival gate are really helpful about directions, especially with such a tight connection. The good news is it's not the same place as 'complete' arrivals and she does not go through the whole check-in process again. (Sorry, I know how to find the luggage place, but can't describe it in a helpful way!)
Have a wonderful time!