Cinda by Janet Lambert

Fourteen year old Cinda Hollister sets out to help her third cousin Paula Marsh. Paula is deeply troubled by her mother's plans to re-marry and Cinda tries to persuade her to accept her new stepfather, Frederick Aldrich. As she plots and plans her strategies, she has plenty of time to enjoy beach parties and many good times with her family and friends. By selling a beloved boat and pooling their savings, the young Hollisters manage to buy their father a second-hand cabin cruiser! Paula sees their unselfishness at work and considers opening her mind and heart to those who love her.