TM.28 Form for Aliens to Notify Their Change of Address or Their Stay in the Province over 24 Hours

Is there a real requirement for the latter part - Stay in the Province over 24 Hours?

Sounds like it could be a gotcha if someone in Immigration took a dislike...

No - it is not normally required. Yes - it can be used as a gotcha.

With the new emphasis on local immigration offices expect it will become more common and it may be a good idea to use for any real change. I would not bother for a weekend stay at the MIL home. (Although it might be a good excuse to not make the trip).