Even being the youthful sister of the future queen of England has its limitations, Pippa Middleton has learned.After her lengthy-awaited first guide, “Have fun: A Yr of British Festivities for Families and Friends,” debuted to not up to stellar evaluations and gross sales, news has come this week that the youthful sister of the Duchess of Cambridge has been dropped by using her literary agent.Middleton’s writer, …