She was born a Black, and intended to remain one.
There was nothing more important to her but blood and family name; naturally, she wanted to remain a scion of one of the most powerful Houses in politics. And to do so, she had to marry one of her cousins.
The eldest objected, very vehemently. “If you want to stay a Black, don’t marry. No one is going to marry a shrew like you anyway.”
So she turned to the younger, more gullible of the two. He had always been little Reg, fading into the background, like an incomplete version of his brother. But now, that malleability was to her advantage. He was easy to bully, to manipulate, to trick into loving her; into thinking that she loved him.
And when she succeeded, the pesky Black pride prevented her from hiding it. She just couldn’t resist showing off her talents, and she burned with a fierce desire to announce her victory.
At the first breakfast of the year in Hogwarts, she laid one elegant hand on Regulus, (with long red nails, strangely resembling a claw), and smiled innocently at the familiar face at the opposite table. When the heavy oak doors slammed shut in his fury, she excused herself from the table and padded out of the room, ignoring the puzzled look the younger boy shot her.
Turning round the corner, she spotted him punching the wall angrily, with blood flowing off his abused knuckles, causing her laugh delightedly. As he swung around to punch her, she raised her hand to catch his fist, and leant forward to whisper.
“Did you see what I did?”