I only have a few details but I would have read this in elementary school. No idea if it would have been new or old then, but def published before like 2000. From what I remember, a kid has a sister (younger?) who goes out in a snowstorm (to protect a lamb? a lamb is involved) but goes missing. Turns out she gets caught in some sort of alterate world/fairy realm/possibly a snow globe?? (I know this sounds insane lol.) But I would love to know if anyone can ID it!
Lost Classics of Teen Lit: Name That Book! 