Their family was divided into quarters, but the quarters weren’t equal.
Regulus, the youngest, had the least responsibility and the least authority. Sirius had a little more, but not enough to be significant.
Their father, head of the household, should have had the most authority. His mouth should have been the mouth to lay down the law, and his hand the hand to deal the punishments.
But their mother, with her unpredictability and her erratic tempers, held all power. And sometimes it felt like her quarter was more like the whole, squeezing the rest of them out into the cold.