Who is Sansa Stark, Alayne from A Feast for Crows and what is their importance?

Sansa Stark is the daughter of Eddard Stark. After her family is cast out and many of them killed as traitors, Sansa manages to escape King's Landing with the help of Lord Petyr Baelish, the former master of coins for King's Landing. They flee to the Vale and the Eyrie, and Petyr marries Sansa's aunt. Sansa poses as Baelish's bastard daughter, Alayne, and waits for her marriage to Harold Hardyng, which Baelish promises will be her chance to announce her true identity and regain her birthright.