Originally Posted by
darthbimmer
Yes, YVR has US customs & immigrations, so she'd pre-clear in Vancouver and arrive at SFO as if she were on a domestic flight.
But which airline is she booked on for each leg? It matters which terminal she lands in and departs from at SFO, as only a few terminals are connected air-side. The rest require reentering through security.
My guess is that YVR-SFO is United or Air Canada, and SFO-TPE is on Eva Air, which will all be airside within International-G. If by chance the YVR flight comes into Terminal 3, there is an airside bridge between T3 and I-G.
Worst case, the TPE flight is on China Airlines out of International-A. That would be a bit of a walk or AirTrain ride and re-clearing security in I-A. A two-hour cushion is more than adequate for that, unless the OP's mother has serious mobility limitations.