OP , you say your sister is a US resident but what passport does she hold?
As
Often1 said, cross border flights right now in many directions are ineligible for complete OLCI due to the many document checks required.
Sorry to say, but Health Canada has even updated the COVID-19 travel restrictions website a few weeks ago stating that they have no compassion. Sorry about your mother. If your sister is a US citizen she will likely be not permitted. If Canadian, then no issue excepting the isolation period.