1. It depends on the officer how he considers your situation as intention to reside in Montreal or Toronto. As your destination is toronto and your sister also lives in province other than Quebec, its easy for the officer to decide about your intention. So i would say that you should either have your final destination to be quebec and have the trip till that point(irrelevant of port of entry in toronto/montreal). But make sure to tell that you are completing the landing formalities now and will move after some months or next year.
2. Dont give your sister's address to get your PR card for the same problem mentioned in #1. Arrange for p.o. box address or plan to get the pr card later. Getting PR card is not priority as the priority to have smooth landing to confirm the PR.
3. THey process to confirm the PR. Sometimes officer wont allow someone else's address to process application for PR card but its fine as you can request later.
SIN: You need to go to different office to apply for SIN. This is secondary as you can get this after you move. This wont be necessary till you join some company locally.