((Changed the Time Line so thing can match up. Sirius didn't runaway from his Family later on, but instead earlier on. He ran away in his 2nd year at Hogwarts. And because of the Heck of it let's have Barty Crouch Jr. start at Hogwarts the same as Regulus, instead of the suspected time we think he did in 1973. Barty starts in 1968 instead. So he's in 4th year right now because it makes more sense.))

It was a New year at Hogwarts. It was September 1, 1972. Regulus is starting his 1st Year at Hogwarts. Sirius is starting his 2nd Year, though he wasn't home during the Summer as Your guys Mother was furious that Sirius was a Gryffindor. Breaking the Long Tradition in the Black Family. She wanted nothing to do with him. You, being the Eldest Child and Starting your 6th Year, took most of the heat from your Mother. You are a Slytherin, but Sirius wasn't.

It is the first week back to Hogwarts, all the New First years were sorted The first night everyone arrived, Regulus, of course, got into Slytherin.

Over the Summer Your mother Had told you that after Hogwarts you will be Marrying Lucius Malfoy, in an arranged Marriage, your Cousin Narcissa Black was supposed to be the one to marry him but your Mother Said you were a better 'match' for him, who was a 7th year. And a Slytherin Pureblood. Lucius is also The Slytherin Head Boy. You and him were just 'dating' right now, enough you both are technically Fiancé's. You both had already spent time together over the Summer, and had to get to know each other.

Since Lucius is The Head Boy Of Slytherin, he gets his own dormitory. And you stay in their with him. Lucius was called The 'Slytherin Prince', and You were called The 'Slytherin Princess'.