SOLVED. Middle grade chapter book about a girl whose older sister was a runner and disappeared in a whirlpool [s]

[Sarah]

I have a student who remembers reading a chapter book with a fox and snow on the cover, which was about a girl whose mother had already died (while carrying in groceries), and whose older sister has now disappeared. The family thinks the sister, who was a runner, may have fallen into a whirlpool, which they'd warned her not to go near. The police are investigating, and the living sister finds her missing sister's belongings in a cave.

My student didn't get a chance to finish the book, so she doesn't know what happens next.

[Andria]

Maybe a Fox

[Sarah]

You’re a genius, Andria!!! Thank you so much!

[Andria]

You're welcome! Happy to help :)