My kids' first "name" words, besides Daddy and Mommy, were Sadie and the name of their beloved foster cousin.
Do they miss Sadie? I honestly don't know. I think they noticed that she's not here any more, but she didn't interact with them much, and I'm not sure that it's really affected them in any way.
Their cousin, though... he was one of the first people they really noticed. He was small, like them, but even less mobile and had awesome hair. They've always been fascinated with his hair - when they were first starting to crawl, they'd make a beeline for wherever he was so they could touch his hair. I mean, it's not like they had much (any) of their own...
They loved seeing him so much that I made a little quest of finding them a soft, cuddly doll with his coloring and, especially, similar hair. We called him Baby until one day Eleanor proclaimed him to be named after Little Man.
She loves her "Baby Little Man" almost as much as the real one - their cousin is their little buddy and partner in crime in every way. I think sometimes it's hard for twins to integrate another child into their way of playing - I guess that's probably true of all toddlers, but sometimes it's more surprising of twins, since they're so used to playing with each other. But a lot of the time, if we add another friend to the mix, they just don't know what to do with them. Even the things they do together every day are just different when they try to do them with someone else.
Not with Little Man, though - he's their third amigo, their third musketeer. They do things together, and don't get mad at each other, and every time they see him, or a picture of him, or his namesake doll, they shout his name gleefully. I frequently wake up to hear them over the baby monitor, having lengthy conversations with each other and imaginary Little Man.
Now, though, it looks like it'll soon be time to say goodbye. I'm not sure how to do that. I'm not sure how to do it for me, as an adult, much less how to prepare or explain it to my babies. A part of me wishes that they had a little more language comprehension, so I could explain it, but another part of me thinks... I don't even have the language to explain it yet. As they get older, I know I'm going to have to find a way to help them understand fostering, and why their cousins might come and go, and why we love them just as much whether they're here for now or here forever. I do wish they had the language (and who knows, maybe they do and we'll find out) to let us know that they miss him. Will they miss him? Yeah, I really think they will. They'll have other cousins and other foster cousins, and other best buds, but for right now, for this season, it'll be incredibly hard for all of us to say goodbye.