Are Cortana and Master Chief in love?
Yes and no. They are not in love in a romantic sense, but they do love each other.
Halo 4 serves as the best exploration of the relationship between the two of them and really shows how they see one another. They rely on each other constantly and for everything and as they face all of their struggles, they do it together. Cortana has to face her own madness and Chief has to not only face the biggest and most unknown threat he ever has, but he has to do so while his best friend struggles for her life.
Josh Holmes, the creative director for Halo 4 and the Halo franchise, has classified their relationship as best friends on several occasions  . And while they don't shy away from calling it love, and a love story, they also don't refer to it as anything more than platonic love.
That said, Microsoft as a whole doesn't seem to be as willing to commit to what to call it. They even poke fun at it themselves with their new Cortana voice command system.
So yes, the Chief and Cortana are in love with each other. But indications from 343 at the moment are that it is platonic, not romantic love.
You could say "maybe" there is no "official" answer - I don't think one will come anytime soon BUT in the sequence of Halo 4
Cortana leaves chief by saving him in the end game. CUTSCENE