Optimize Conditions

I've a simple question about conditions in if and for or while loops. Is there any way that make me verificate this condition with less lines of code?

if (are_you_sure != "Si" && are_you_sure != "si" && are_you_sure != "No" && are_you_sure != "no")

I don't think i can minimizate the code above, but I would like to be sure.