Professor Carol Propper of the University of Bristol and colleagues in Norway examined data of children born between 1947 and 1958 to compare families from a similar background from different regions of Norway.
The researchers said the probability of the younger sister having a teen pregnancy doubled to two in five if the elder sister had a baby as a teenager, the BBC reported.
Teens who spent longer time in school were less likely to become pregnant as a teenage, but this was on a smaller scale than the "sister effect," Propper says.
"Sisters generally spend more time together than schoolmates or friends and so sisters are likely to be influenced by the behavior of their siblings," the study says.
Propper said two groups of girls are particularly vulnerable to the "contagious effect of teen motherhood," those in poor households and sisters who are close in age.