There Will Be Blood

In 1898, Daniel Plainview, a prospector in New Mexico, mines a potentially precious ore vein from a pit mine hole. In the process of dynamiting the lode, he falls from a broken rung of the tunnel ladder and breaks his leg. He saves a silver sample, climbs out of the mine, and drags himself to the nearest assay office to evaluate his find and receives a silver and gold certificate claim. In 1902, he discovers oil near Los Angeles, California and establishes a small drilling company. Following the death of a worker in an accident, Daniel adopts the man's orphaned son. The boy, named H. W., becomes his nominal "business partner", allowing Daniel to paint himself to potential investors as "a family man".
In 1911, Daniel is approached by Paul Sunday, who tells him of an oil deposit under his family's property in Little Boston, California. Daniel attempts to buy the farm at a bargain price. However, Eli, Paul's twin brother and pastor of a local church, is aware of his plan. Eli demands $10,000 and states that it is for the church. An agreement is made and Daniel goes on to acquire all the available land in the area, except for one holdout: William Bandy. Oil production begins, but an on-site accident kills a worker and a gas blowout robs H. W. of his hearing. Eli blames the disasters on the well not being properly blessed. When Eli demands the $5,000 Daniel still owes his family, Daniel beats and humiliates him. At the dinner table, Eli berates his father for trusting Daniel.
A man arrives at Daniel's doorstep, claiming to be his half-brother, Henry. Daniel hires Henry to work for him, and the two grow closer. H. W. sets fire to their house, intending for it to kill Henry. Angered by his son's behavior, Daniel sends him away to a school for the deaf in San Francisco, and leaves him on the train by himself. A representative from Standard Oil offers to buy out Daniel's local interests, but Daniel elects to strike a deal with Union Oil and construct a pipeline to the California coast, though the Bandy ranch remains an impediment.
While reminiscing about his childhood, Daniel becomes suspicious of Henry and one night holds him at gunpoint. "Henry" confesses that he was a friend of the real Henry, who died from tuberculosis. In a fit of rage, Daniel murders the impostor and buries his body. The next morning, Daniel is awakened by Mr. Bandy, who knows of the previous night's events and wants Daniel to repent. At the church, as part of Daniel's baptism, Eli humiliates him and coerces him into acknowledging that he is a bad father. Some time later, as the pipeline is well under way, H. W. returns and reunites with Daniel, while Eli leaves town to perform missionary work.
In 1927, H. W. marries Eli and Paul's sister Mary. Daniel, now extremely wealthy but a raging alcoholic, lives as a recluse in a large mansion. Through a sign language interpreter, H. W. asks Daniel to dissolve their partnership so that he can establish his own oil company in Mexico. Daniel reacts brutally, mocks H. W.'s deafness and reveals his true origins as an orphan. H. W. leaves, but not before telling Daniel he is now grateful that they are not related.
Soon after, Eli visits Daniel, who is drunk and passed out in his private bowling alley. Eli, now a radio preacher, offers to sell Daniel the land of William Bandy, who recently died. Daniel agrees on the condition that Eli loudly denounces his faith and his own credibility. Swallowing his pride, Eli reluctantly does so. Daniel reveals that the property is now worthless because he has already drained its oil through surrounding wells. Shaken and desperate, Eli confesses to being in dire financial straits. Daniel taunts Eli and claims that Paul now has his own oil company. Daniel then snaps and begins attacking Eli, and chases him around in the bowling room. Eventually, Daniel kills Eli by smashing his head in with a bowling pin. When Daniel's butler comes down to check on him, Daniel says casually, "I'm finished."

Question: Who is Eli and Paul's sister?
Answer: Mary