Children’s Book about Peggy Shippen’s Seamstress

I’m looking for one of my mom’s favorite childhood books. She’s not 100% certain about any of these details.

It was most likely published in the 70’s, and no later than 81. The main character is named Hannah. She lives in Elfreth’s Alley in Philadelphia, is in her late teens, and is a seamstress. The plot kicks off when she’s commissioned to make clothes for the Shippen girls. She gets secret information, probably about troop movements, out to Valley Forge, and Major Andre is a fairly important character. She has a romance with a guy named Mark, who she’s known since childhood and who’s a soldier in Washington’s army.

Many thanks for any suggestions you have!