Originally Posted by
tentseller
What airline are you flyng into YYZ and out of?
There is airside transit US arrival to intl departure in T1.
My guess on your intinerary is that you are flying into YYZ with DL who arrives into T3 which does not have airside transit. So you have to enter Canada and be admissible.
Even if all flights were in and out of T1, there is still a passport control with Canadian Immigration. It requires luggage tagged all the way to the final destination and a boarding pass for the onward flight.
That said, I don't know how they would react to the OP's sister's situation, even if just in transit.