Agreed, I have literary never heard anybody use "hvis" as "whose". "Hvem sin" is the term that should be learned here. I think most Norwegians wouldn't even understand you if you used the phrase "Hvis mat spiser vi?"
Totally agree.. Never heard that before, so a report is sent!
It would be funny if someone sat besides me and asked me "If food are we eating?".. hahaha.. I will try it on my friends and see how they react..