Misty eyes will be the way of it for a while -- share them with the kitty who's looking around corners...
I heartily concur with all that's been said. And, like humans, they each have their own way of dealing with their grief.
When our Samantha disappeared several years ago, her litter mate, Suzy, was just as keenly looking for her as we were -- to the point, one evening, of leading me from the back porch, down the stairs, across the back lawn, through the gate, across the lane, to a neigbour's carport, where there was some furniture under a tarp, which she sniffed around minutely, looking at me frequently. You tell me -- was she saying that Sam had been there, trying to give me a lead to follow? I think so. Not that it took us anywhere useful, but she sure tried.
And when wonderful neurotic old Nibs (Grampy Cat)finished his innings the following summer, leaving Suzy on her own, Sooz would also wander around, stopping in his favourite places and look up at us with the most soulful eyes, and meow with a definite question mark. It was harder to give her the extra cuddles we wanted to, because she is such a reserved kitty, and really doesn't like to be mushed with most of the time.
But you do what you can for each other -- and that's probably the key: for each other.