I've seen some posters recently with amazing skills at identifying books from very vague clues so I thought I would try this one which I have been trying to track down for years.
This is very, very vague, apologies and some of the details might be misremembered.
A girl (and her sister?) go to stay with their grandmother over the holidays, but it's not the gran they normally stay with - it's their other gran and she lives somewhere foresty and a bit witchy.
Their parents are separated and she really misses her dad.
There's some kind of magical artefact in the book - possibly a necklace - that is lucky or grants wishes.
At the end of the story she is reunited with her dad but he only seems interested in his step-family who are boys, and essentially rejects the girl. The ending is very sad - I wept at it as a child.
It was definitely the early 80s when I read it but it was a library book so might have been older than the 80s.
If this rings any bells I will be amazed and grateful.