From my observations the passport check is a visual check by an uniformed CBSA agent at Intl/US arrival in T1. There is a counter with the full CBSA terminal there but they are usually in front and just looking at the passenger and the passport.

I am guessing that OP will be arriving in T3 due to DL is the only airline with acheduled flights from DTW to YYZ. If my hunch is correct, her sister will be subjected to full immigration arrival including scan of the passport to transit.