My parents in law live at my cottage and don't pay rent (but do cover some expenses like hydro). Should I treat my cottage as a rental propert without revenue?

No.

Rental properties need to be rented out at fair market value to be considered a rental unit. In your case, it would be considered personal use property and subject to capital gains when you sell.

Remember that ALL capital expenses including renovations are added to the original cost to increase the Adjusted Cost base to keep the capital gain lower.