Four Brothers

The seemingly random murder of their adoptive mother, Evelyn Mercer (Fionnula Flanagan), at a Highland Park, Michigan convenience store, brings four brothers back home to Detroit, Michigan to find out what happened. Originally under the impression the crime was a simple robbery-gone-wrong, the brothers soon discover that the robbery was merely a cover for what was, in fact, a hit put out on Evelyn. After this revelation, Bobby (Mark Wahlberg), Angel (Tyrese Gibson), Jeremiah (AndrÃ© 3000) and Jack Mercer (Garrett Hedlund) track down the hired guns who killed Evelyn. Refusing to say anything, they are unceremoniously executed by Bobby and Angel.
The next day, Detroit Police Lieutenant Green (Terrence Howard) and Detective Fowler (Josh Charles) confront the brothers about the murders. Lieutenant Green warns them that their interference with Evelynâs case is ill-advised, and that it will eventually put them in over their heads. After confronting Jeremiah about the revelation of his failing business and benefiting from Evelyn's life insurance, the brothers are treated to a somewhat different version of events. Jeremiah informs them that his construction company was failing precisely because he was not getting involved with Victor Sweet (Chiwetel Ejiofor), and that for a project to succeed he had to pay off the right people, which he initially failed to do. In his effort to restore his business and relieve pressure from himself, he tried to pay off Sweet's henchmen. As for the life insurance, Jeremiah explains that the money went directly to him for his daughters, because he paid all of Evelyn's bills while his other brothers were not around.
Back at their home, during a confrontation with Jeremiah, Sweetâs men attack the brothers. Jack is shot and killed during the attack. Bobby finds one of the gunmen still alive and questions him about who sent them. He almost spares the gunman but is offended when the gunman mumbles "Thank God" and kills him anyway. When Lieutenant Green arrives, he tells them not to worry about any legal ramifications, assuring them that it will go down as self-defense. He also informs them that Evelyn filed a police report regarding Victor Sweet and his involvement in Jeremiah's affairs, and his partner, Detective Fowler, passed on that report to Sweet. Green warns the brothers to stay out of the matter and let him handle Fowler and then they will work together on Sweet. Later at a bar Green confronts Fowler, hitting him and ordering Fowler to hand in his badge. They walk out of the bar, and Fowler kills Green and calls it into dispatch claiming two assailants had fired upon Green.
Meanwhile, the remaining brothers devise a plan to buy Victor Sweet off with the $400,000 from their mother's life insurance. When Sweet accepts, Angel sets off for Fowler's. Arriving at Fowler's, he subdues him. Jeremiah then goes to meet Sweet, while Angel's girlfriend, Sofi, heads to the police station, where she tells the police that Angel is planning to kill a police officer. Hearing the sirens in the distance, Fowler thinks they are coming for Angel, until Angel opens his jacket showing a wire. Angel claims the whole conversation was taped, including Fowler's admission that he killed Green. The police arrive at Fowler's in full force, and Fowler gets the upper hand on Angel. With a gun pointed to Angelâs head, Fowler tells the police to back off. Fowler opens fire on the officers outside, who return fire and kill him.
Meanwhile, at frozen over Lake St. Clair, Jeremiah meets with Sweet and reveals that the $400,000 is to buy off Sweet's henchmen and kill Sweet in exchange. Sweet angrily demands to know who will be the one to kill him just as Bobby shows up. Bobby and Sweet brawl, resulting in Bobby knocking Sweet unconscious. His former henchmen seal his fate after he is dropped in a hole carved into the ice, drowning him. The three brothers, taken into police custody, are beaten in an attempt to make them confess to the murder of Victor Sweet, which they do not. Back home, they set about repairing their mother's house, and continuing their lives together.

Question: What reunited the four brothers?
Answer: Evelyn's murder