YA book. Two boys and a man in a giant tree house spend the summer in a rivalry with 2 girls and an older woman. Stand alone book. Read in early 2000s, but may be 1990s book.

[Jan]

Read this in my old High School library in early 2000s

[Rainbowheart]

Were the boys teenagers or younger children?

Did they live in the treehouse, or just hang out in it? Who was the man, an adult relative or friend?