As to who the title, The Other Boleyn Girl, refers to; it varies. when Mary was the King's favorite, Anne was. when Anne was favored, Mary was.
The story is told from the P.O.V of Mary (in the book she's the younger sister, historians think she was the older) who was married at 12 and at 14 was commanded by her family (mostly her mother's brother) to become Henry the 8th's lover when he shows an interest in her. The Howards, her mother's family, have been serving the king for years & are looking to get ahead, and if that means throwing their daughter(s) into the tangled web of court politics, so be it. Anne points out that there are a lot of girls in the family, so if isn't them being used as tools it will be someone else. In fact the Howards will repeat this folly later by making Catherine Howard (a cousin of Anne & Mary's) Henry's 5th wife, which results in her beheading for adultery. The Howards are out to keep ahead, by getting favors & titles, of the Percys & Seymours. The Seymours eventually get ahead by placing their daughter Jane on the throne as henry's 3rd wife. (she dies after giving birth to the future king Edward the 6th)
In the novel Mary reluctant to have an affair with the King since she is married, but comes to love Henry. She eventual gives birth to two children by him; Catherine & Henry. Historians doubt Henry was the father of them, he never acknowledged them as he did with his son by Elizabeth "Bessie" Blount. Maybe it's because Mary was married & Bessie wasn't? The King later gave Anne wardship of Mary's son after the death of her husband. Why would he give wardship of a boy to an unmarried woman, unless it was his son & he was planning to marry her and make him his heir? It's interesting to note that Henry's initial attempts to obtain a papal annulment of his marriage to Catherine (his 1st wife), he also requested a dispensation to marry his mistress' sister.
Mary managed to escape the madness of court politics by marrying a second time, for love, and going to the country to be a farmer's wife. A station so far below her that Mary's family, Anne included, disowned her. She had two more children; Anne & Edward and lived the rest of her life in obscurity.