She could have had him. She had had him at one point or another. But she couldn't have her, so to take him would mean breaking her heart. And Echo could no more hurt Kit then she could cut out her own heart.
So she'd taken the middle ground. If she couldn't have Mat and Kit as lovers, Echo would accept love of the sibling sort. And some nights, when Kit woke up crying or Mat screaming or Echo had spent half the night vomiting her disgust at a job, or all three of them were frightened and hurting, she could snuggle between her loves and pretend that being their sister was enough for her.
Some nights, when her bed was empty and the nights long and filled with far too much thought, Echo hated them both for not seeing what it was she needed so badly. But love wouldn't allow her feel such a thing towards them for more then a moment or two. So she had been their friend, their sister and eventually, their savior.
Echo remembered the chill, and the peace, of death. For awhile the ache of being forever without what she desired seemed to fade. But it was then that she discovered how much deeper Loneliness cut compared to the sting of Loss. She had no one to laugh with, to cry with or to play with. There was no one to make her smile or to make smile. And the wait between emptiness and peace seemed endless.
But when they at last did arrive, the time seemed not to matter at all. The pain of Loss and Loneliness was finally forgotten as Echo took her place, safely in the middle once more.