Final answer:
In the book "Shiloh" by Phyllis Reynolds Naylor, Marty knows that Sam Wallace would spill the beans about Shiloh.
Step-by-step explanation:
In the book "Shiloh" by Phyllis Reynolds Naylor, Marty knows that Sam Wallace would spill the beans about Shiloh. Sam is Marty's best friend's brother who has a reputation of being a blabbermouth, so Marty knows he can't keep a secret.