Ben Braeden is the child of Lisa Braeden, his mother. A young Nicholas Elia plays him in Season 3, as we see a boy that shares several similarities to Dean, in particular his haircut, apparel and love of classic rock music.
Sam and Dean actually do have one more sibling, a younger brother named Adam. Adam Milligan, the half-brother to the Winchester brothers. John Winchester fathered Adam sometime after Mary passed and he was on the road hunting a lot. He met Kate Milligan, Adam's mom, and the two had a child together.
