A possible concise summary is:
Marin, a grumpy teenager who feels out of place with her cheerful and famous parents, finds out she is adopted and has a sister named Franchesca. Franchesca visits her and seems to be perfect in every way, but Marin discovers she is part of a scheme to rob her parents. Franchesca changes her mind and confesses, and Marin's parents adopt her. Marin also realizes she has feelings for her best friend Ray.
Here are the key facts extracted from the text:
1. The narrator, Marin, is from Vermont.
2. Marin's parents are a psychologist and a life coach.
3. Marin's parents made millions and became famous selling books on happiness.
4. Marin's parents are always cheerful and embarrassing.
5. Marin is 16 years old and doesn't share her parents' cheerful disposition.
6. Marin's parents told her she's adopted when she was 2 years old.
7. Marin has a sister, Franchesca, who is 2 years older and was adopted from the same orphanage.
8. Franchesca contacted Marin after seeing their parents on TV.
9. Marin and Franchesca made plans to meet, and Franchesca visited Marin's home.
10. Franchesca is popular and well-liked, but Marin is initially wary of her.
11. Marin's best friend, Ray, is strange but lovable.
12. Marin and Ray spy on Franchesca, who is suspected of being a liar and up to something.
13. Franchesca is caught speaking in a coded language and is overheard discussing a plan to steal money.
14. Marin confronts Franchesca, who reveals her difficult childhood and her plan to steal money.
15. Franchesca's foster parents are arrested, and she moves in with Marin's family.
16. Marin and Franchesca become close, and Franchesca becomes a part of the family.
17. Marin and Ray are pushed into a pool by Franchesca, who teases them about their potential feelings for each other.