Think Positive!
Frannie & Larry Shoemaker are again camping with friends at a Minnesota Campground near the Burden River noted for bicycle trails, nature center, and old power plant. Shortly after arriving they meet Virginia and Valerie Peete. The twin sisters are in their 30s and still dress alike. Frannie sees one sister in a romantic stance with Richard Ellis-Reynolds. He denies knowing them. The group hikes to the old power plant and finds the sisters murdered. In an accident getting into a boat, Frannie hurts her knee and must stay off it. The group goes on the hikes, and bicycle trips, and explores the day nature center at Frannie insistent. Frannie has plenty of time to look for clues to the murder. The day before the group leaves the murder is uncovered. The ending is a surprise
idahobluebird50 Plants, Animals, Cats, Dogs, Mysteries and Chat 
I have this book on my TBR pile.
That is a different twist to have both of the twins killed!
Thed only other I read that It was an Ellery Queen book and I haven’t remember which one.
You have a great memory for what you have read.