I was sure was the book was titled “Just Plain Nancy”, as my name is Nancy. It’s about a girl who has a sister, I think named Marjorie. Marjorie was named after her mother’s friend, but “Nancy” was just plain “Nancy”. It was set in the time when “Nancy” could ride along on an ice wagon.
This is “Just like Nancy” sequel to “All about Marjory” by Marian Cumming. Published in 1953.