This is a kids mystery book, probably written in the early 70s. The author’s last name was at the end of the alphabet–Mayba a W.
Two sisters–I think twins, but they may be triplets, where two are identical and the third isn’t. The girls are on vacation in either Newfoundland or Nova Scotia. They run into another girl who looks exactly like one (or two) of them. the non-identical sister is convinced she’s adopted, and that the girl they saw is the actual twin (or triplet) of the others. They try to find the girl again, but she’s been kidnapped.
There’s a whole mystery involving caves and some missing jewelry? The parents are kind of cagey about why they’re In Canada, so they’re not a lot of help, although they are present. They’re at a lawyers office quite often, giving the girls unsupervised time. They use the resemblance between the kidnapped girl and the sister to trap the crooks somehow, with the fathers involvement.
At the end, it’s revealed that the kidnapped girl is their cousin, which is why she looks like them. I think the parents were trying to arrange to adopt her.
I’ve been trying to remember this for awhile, but can’t. Googling just brings up a lot of results, which aren’t helpful.
Thanks for any help!