From Cain and Abel and the Brothers Karamazov to Cinderella, the warmth and support provided by siblings has hardly been taken for granted. Now, researchers have found that children who moan about their brothers and sisters may have good reason to complain: the more siblings teenagers have, the more it hits their happiness, they claim. A study of secondary schoolchildren in the US and China found that those from larger families had slightly poorer mental health than those from smaller families. The greatest impact was seen in families with multiple…