I agree. They are sisters after all. They most care of each other. But we've seen them not quite often this close and the sillyness of the 'sunshine, sunshie' greeting (that I find very cute) doesn't help.
Wonderful work on this. I very like Twilight and the more I see Cadance, the more I like her. But did you have something in mind they would say in this situation and where they are supposed to be?
I really love the way you drew Cadence and Twilight. The way they are holding each other's hooves in a kind yet ambiguous way. I guess they are both admiring each other. Twilight admiring Cadence for always taking care of her as her foalsitter, or her strength and for having responsibility in protecting the Crystal Empire. While Cadence admires Twilight for her innocence, her intelligence, that she saved Equestria more than 3 times, her devotion and that she worked hard enough to become the princess of Magic. And if I her her I would love and admire Twilight too.
I also love the way the light is shining between them in the darkness. It makes it heartwarming. (I heard from psychologists that colors could represent different emotions and moods).
But If I had one bit of criticism It would be better if in the drawing both Twilight and Cadence had tears of joy in their eyes. I guess to strengthen their pseudo sisterly love of each other. But that's just me.