343T: Summer Time Twins (Solved!)

The book I am looking for is a young adult book. I read it when I was a pre-teen in the early 90’s. It had twins, a boy and a girl, and the girl was named Willa. The boy may have been named August. They were also pre-teens. Their mother may have had a baby—a boy?? I remember trees, summer time and perhaps a tree swing or hammock. Maybe lemonade?

233H: Adult fiction with descendants of Jesus of Nazareth

Twins, Roslyn Chapel, descendants of Jesus of Nazareth
Adult mystery about the secret descendants of Jesus of Nazareth. A girl and her grandfather are the main characters. At the end of the book they come to Roslyn Chapel where they go into a hidden underground chamber and where they meet the girl’s grandmother and twin brother, whom she didn’t know existed. It’s not The Da Vinci Code, I must have read it 10-15 years earlier.