"Sibling relationships are the most enduring relationships people have. Parents die and you don't meet your spouse until later in life. So throughout life, siblings really remain important," said Padilla-Walker.
Celebrity siblings like reality TV's Kardashian sisters are a classic example of how sibling support has benefited people who would never have otherwise hit on stardom.
While the sibling factor cuts across racial and economic barriers, it loses strength in single-parent families, the study showed.
"In a single-parent family, the sibling may be taking on a parental role rather than the role of a confidant. The parent-child relationship is vertical, with the parent in charge. The sibling relationship is horizontal, you're on the same ground. That's a good thing because you can talk about what you're really feeling. There is no feeling of authority or being told you're doing something wrong," Padilla-Walker said.
"Once they hit adolescence they have someone to confide in," Padilla-Walker said. "They can get into stuff they don't feel comfortable discussing with others in the house."
The gift of gab may explain why sisters seem to improve their siblings' mental health. Padilla-Walker said previous research has shown that girls talk more than boys.
So having a sister in the house may be similar to having a therapist on call 24/7, even if it means whispering in the dark to the person who keeps shaking the bunk bed.