Summer sister starts out with the main character, Victoria receiving a phone call from her longtime friend, Caitlin. Caitlin calls the then fresh from Harvard 22 year-old Vix (short for Victoria) and tells her she's getting married and that she wants Vix to be her maid of honor. The only problem is that Caitlin is marring Vix's old flame. When Vix hears this news she is suddenly transported back in time and through her old memories. Starting back when Vix and Caitlin first met at 12 years old.
It started... Caitlin has just transferred to Vix's school, and quickly becomes the most popular girl in the school. Vix is somewhat shy and withdrawn, so at first she thinks it is a joke that Caitlin invites her to spend the summer at her fathers house in Martha's Vineyard. It is during this first summer that the two forge an intense and moving friendship. Soon after arriving on the Vineyard, Caitlin encourages Vix to agree to a pact vowing never to be ordinary. The NBO pact.

Blume uses the friendship between the two to explore issues of class and money and how they affect a persons interactions with the world. Vix is from a poor family in New Mexico. Caitlin's parents are divorced, but both are wealthy. Caitlin's mother, Phoebe, spends her summers cavorting in Europe with handsome men, while her father, Lamb, has roots in old money on the east coast.
Caitlin and Vix become so close that they spend every summer of their teen years together on the vineyard. Caitlin's father and stepmother Abby take an interest in Vix, encouraging her to use her intelligence to reach beyond what she might have sought before meeting Caitlin. This leads to problems in Vix's relationship with her family in New Mexico. This relationship becomes permanently strained when Vix is accepted at Harvard and wins a scholarship from the Soiners Foundation.
Vix is caught between two worlds at Harvard. Her life experiences make her much more open and tolerant than many of her fellow classmates, yet she wants to fit in with her classmates as well. Vix also has trouble at first because Caitlin has gone to Europe and decided to live there for awhile. Caitlin stays in Europe even as Vix graduates and moves to New York City, a move that costs Vix her boyfriend of five years. It is only later that Vix finds out why Caitlin stays away.
I really enjoyed reading the novel by Judy Blume, the characters are real and completely believable. The main characters are teenage girls, and that's what drew me to this wonderful book. Blume uses an interesting style to get everyone's point of view across during situations, and in doing that she helps you see the whole picture. Blume can put down on paper thoughts that every teenage girl has but sometimes is too embarrassed to say out loud. I found Vix's thoughts about sex and love extremely real, and was glad to find that I wasn't the only one thinking that way. This novel made me feel proud to be a girl. The story line was so interesting and fun that I read the 200+ page book in only 3 days. I found myself to be laughing and crying at the same time sometimes. I am glad I choose this novel to read because it made really think about some of my friendship, and how great they are, or how I can improve them.