Short story: boy who speaks a dead language

I read a short story some years back about a boy who was apparently born speaking a language nobody had heard before. Later on in his life some linguists came along and identified it as a language that had died out completely shortly before and no longer had any living speakers--other than this child. Throughout his life he was either unable or unwilling to learn any other language. I seem to recall his (perfectly normal) sister grew up and studied neuroscience or linguistics or something in order to try to understand him better.

I think it might've been in one of Gardner Dozois' anthologies, but I'm not sure.