I read this book maybe fifteen years ago. It may have been contemporary for the time or a bit older. I remember it referenced a number of old fairy stories, although I wasn't familiar with them at the time so I didn't recognize the allusions. As I recall, the words "fairy" and "changeling" are never actually used.
The plot involved a teenaged protagonist finding a small boy or toddler and taking him home to her family, which definitely included two or more brothers. The child is strange/inhuman in some way, and I think he has color-changing eyes. The girl is very taken with the little boy. I seem to remember that she liked him better than her brothers, but that might have been my impression rather than something textual.
At a certain point she becomes disenchanted with the boy. The climax definitely involves shapeshifting. I'm almost certain that she has to hold onto him while he changes shapes -- the fairytale connection I was talking about. I wouldn't have recognized this reference at the time.
I did read it at around the same time as Dianna Wynne Jones' Fire and Hemlock, which has some similar themes, but I don't think they're otherwise related.