Double Date by Rosamond du Jardin

Quiet, sensitive Penny Howard has always tried to be as much like her vivacious twin Pam as Pam wished- wearing the same clothes, and letting Pam arrange dates and choose their activities. But as the girls start their senior year at a new high school, in a new town, Penny resolves to no longer be a carbon copy of Pam.. To her surprise, she finds herself quietly cheered on by their mother, and their grandmother, and most happily for Penny, by Mike Bradley, the boy she was afraid Pam had chosen for herself.