The Black Family was a family of dark wizards. Mr. and Mrs. Black beilved that to be Black made you practically royal. Regulus beilved them, but Sirius did not, and wasn't a dark wizard like them. Mrs. Black swore that Sirius was no son of hers and Sirius was constatly reminded what a better son Regulus was.
At age 16, Sirius ran away and went to live his school friend, James Potter. When Lord Voldemort rose to power, Regulus joined him. Mr. and Mrs. Black thought Regulus was a little hero for becoming a Death Eater. When he had gotten in so far, he began to panic and wanted to quit, but being a Death Eater is a lifetime of service of death. Regulus was killed on Voldemort's orders.