The Szyslak Brothers were a crime family.
When Ned Flanders was young, his grandfather, Sheriff Flanders, was at Lachman's Barber Shop and Delicatessen getting a shave when the Szyslak Brothers entered. Slocum Szyslak, the leader of the group, told Sheriff Flanders that he was going to get what was coming to him. Sheriff Flanders sent the barber and his grandson out of the shop so he could receive a bribe from Slocum. Slocum then told Sheriff Flanders that the bribe was bigger than usual as they had a deal with the Capital City Mafia to deal in amphetamines. After he received the bribe, Sheriff Flanders threw Scrotum Szyslak through the window to keep up appearances.
Later that day, Sheriff Flanders went to parties with the Szyslak Brothers and eventually they ended up at the Prestige Motel where the amphetamine deal was happening. However, the Szyslak Brothers decided to double-cross the Capital City Mafia and a huge fight broke out that led to the deaths of everyone except Slocum, Kostas Becker, and Sheriff Flanders. Sheriff Flanders killed Slocum to take the money, and wiped out the last of the Szyslak Brothers.