Hygro
soundcloud.com/hygro/
The study specifically mentions that numerous other studies discussed the role of parents in shaping the politics of their children. But this is apparently the first study which tried to assess the role of siblings in determining their political outlook.
But it doesn't discount the role of parents in doing so, unless I'm assuming too much (little) of the study. As in, the known variable is siblings, but the uncontrolled variable is a change in parental behavior because there are girls, and the effects that parental effect has on the boys witnessing it.