Still of James Floyd and Fady Elsayed in My Brother the Devil. (Simon Wheatley)

Most of the young toughs in “My Brother the Devil” call each other, in their nearly impenetrable East London accents, “cuz,” “bruv” and, most often, “fam.” Short for cousin, brother, and family, they’re terms of casual endearment that identify the speakers and those spoken to as members of an affinity group that, more often than not, does not include blood relationships. But for the members of the the teenage gangs around whom the drama revolves, those connections forged on the street — and the loyalties, betrayals and prejudices that they engender — can sometimes loom larger than actual family ties.

That’s the theme explored by writer-director Sally El Hosaini’s promising debut feature. “My Brother the Devil” follows two teenage brothers from a family of Egyptian immigrants living in London’s Hackney neighborhood. Rashid, or Rash (James Floyd), is a handsome, 19-year-old delinquent and member of a drug gang who is idolized by his 14-year-old brother Mohammed, or Mo (Fady Elsayed), a good student and a sweet kid who, as the film opens, has so far managed to stay out of trouble. Rash wants Mo to stay in school, go to college and follow a path other than his own.

The good brother-bad brother set-up is familiar. But then events occur that wrench the story out of its predictable path. After the murder of Rash’s best friend (Anthony Welsh) and the subsequent cycle of revenge shake Rash’s commitment to his street family, Mo finds himself getting sucked into the same pernicious lifestyle that his older sibling is now trying to get out of. The gang’s gravitational pull also tears at Mo’s connection to Rash.

Despite a certain ambiguity as to which brother is the “devil” of the title, El Hosaini hints strongly that it’s Rash. The film is, on one level, the story of a fallen angel who is trying to pick himself back up.

But El Hosaini’s story is a bit more complicated than that. As Rash wrestles with the more familiar demons of the criminal underworld, he meets a hip older photographer, Sayyid (Said Taghmaoui), whose friendship tests not only Rash’s relationship with the increasingly jealous Mo, but Rash’s understanding of himself.

It’s a twist that gives “My Brother the Devil” a fresh contemporaneity.

There’s lots of grit and vinegar in El Hosaini’s film, which has the unpolished texture of cinéma vérité. That helps temper the slight flavor of melodrama that sometimes seeps in. There’s a sweetness to the film, to be sure. But like the troubled brothers at its center, the film has good heart beneath its gruff exterior.

Unrated. At Landmark’s E Street Cinema. Contains obscenity, sex, drug use and violence. 111 minutes.