Lending new meaning to the term “big brother”: A recent study of more than 10,000 families finds that siblings are even more powerful than parents when it comes to predicting kids’ weight. Having an obese brother or sister makes a child about five times more likely to be very heavy too, say the study’s researchers, from Massachusetts General Hopsital, Cornell, and Duke University.

There are some obvious potential explanations: Siblings grab snacks out of the same pantry, and usually sit down to the same meals. They often play together, in either sporty or sedentary ways. But the data suggests that some psychological factors may also play a role. Younger siblings are most influenced by an older sibling’s weight if that big brother or sister is the same sex. Kids really do model themselves after the elders closest to them, in so many ways.

So if one child is gaining more weight than is healthy, what can you do to help him—and in turn, his impressionable brothers and sisters? Rather overhauling the heavier child’s diet, take a close look at what you’re feeding the whole family, says Suzanne Rostler, R.D., a nutrition specialist in the Optimal Weight For Life (OWL) clinic at Boston Children’s Hospital, and co-author of Ending the Food Fight. “All family members should be eating in a healthy way, regardless of where they fall on the weight scale,” she says. While sibs may be the focus in this study, she adds, the onus is really on parents, not children, to set family food trends, such as eating mostly fresh, whole foods and limiting highly processed snack foods. (A good rule of thumb: If a snack food has the shelf life of years, it probably isn’t good for your body, regardless of fat and calorie content, Rostler says.) “These habits will trickle down to the oldest child, which will trickle down to younger children,” says Rostler. “And the more you can model good habits, rather than talk about them, the better. A lot of kids we see feel ‘the more you tell me to do this, the less I want to do it.'”

And what if one child has developed a newfound interest in a healthy habit, like eating fruit salads or riding bikes? Excellent: Encourage her to let her sib join in. “Just make sure you don’t frame it as, you (or your sister or brother) need to do this to lose weight,” says Rostler. “Food is emotional, and a comfort for many kids and adults too. If a child is feeling bad because of weight, you risk making them turn to food to feel better, and the cycle continues.”