Both of the Corsican brothers are well dressed men smelling of vaseline, differing in minor details. Marty wears his hair slicked back and has a tendency to sweat, while Sid wears a toupee and dark glasses. However, their personalities are identical and interchangeable, a factor of them being identical twins.
The two are in charge of producing pornographic movies for wasteland consumption. They've been doing this ever since they stumbled onto a fully furnished porn studio, with a QEWB drive and full holographic recording set up. They operate under the Mordinos' aegis, paying them tribute in exchange for protection.
The player can star in their movies, but it requires good Charisma or a Made Man/Prizefighter reputation, as well as high Endurance, Charisma, and Agility.
The fact that there is only one of the brothers at any given time is likely a reference to the 1941 movie The Corsican Brothers starring Douglas Fairbanks Jr. as both of the two Siamese twin Corsican brothers, separated at birth.
The Brothers and the titles they list are a laundry list of references to popular movies.