The customs officer is asking two questions:

1) is this person importing something they have to pay for or should not?
2) is this person admissible to Canada?

If the customs officer does not know the answer to #2 or it is not clear the person will be diverted to immigration. You mother-in-law should be made aware that might happen.

In the letter to customs/immigration it might be helpful to explain the specific dates your sister's mother-in-law is traveling and attach proof of a departing flight. If an officer is convinced your mother-in-law will only be staying a brief time she should be allowed through without the delay of being processed by immigration.

On that note I believe at YYZ it is likely there will be a spanish speaking customs agent.