(NBC News) The young sisters played outside together every day that summer in a Vallejo, Calif., suburb, and during one long, boring summer day, the older girl thought of a new way to pass the time. She led her little sister to the backyard, tethered the 9-year-old to one of the poles on the back porch, then taped her eyelashes to her eyebrows and left her there to stare into the bright California sun. That's the account of Nancy Kilgore, now 63 and living in Sacramento, who says she doesn't remember feeling physical pain that day. She just remembers feeling helpless. "I just basically got to a point where I accepted anything my sister said or did," says Kilgore. Back then, Kilgore didn't know she was a victim, but she says she believes now that she grew up as a bullied sibling, a group that's drawing new attention from doctors and researchers who warn parents that such childhood battles may not be as benign as many believe. The anti-bullying movement has exploded in the last decade and a half — every state but Montana has now passed some sort of anti-bullying legislation, the first being Georgia's in 1999. Bullying as it's typically understood takes place between unrelated peers — it's kids being cruel at school, taunting each other or beating each other up. But a new study published Monday in the journal Pediatrics is a reminder that bullying also happens at home, between brothers and sisters. And, as with peer bullying, sibling bullying is also harmful to a child or teenager's mental health, the new research finds. "Historically, sibling aggression has been unrecognized, or often minimized or dismissed, and in some cases people believe it's benign or even good for learning about conflict in other relationships," says Corinna Jenkins Tucker, lead author of the paper and an associate professor of family studies at the University of New Hampshire. "That's generally not the case in peer relationships. There appears to be different norms for what is accepted. What is acceptable between siblings is generally not acceptable between peers." Of the children interviewed (or interviewed by proxy), 32 percent reported experiencing at least one type of sibling victimization in the past year. Researchers found that "all types of sibling aggression, both mild and severe, were associated with significantly higher distress symptom scores for both children and adolescents," the study authors write. Brother, Sister Fight imageAmy Mikler