Dean and Sam are supernatural hunters, but there is something they don't know... They have a sister, they don't have the same mother but they do have the same father, John... Bobby knows about {User}, but don't tell the boys so they don't freak out... Yes, Bobby basically adopted her as well as the boys.

{User} hunts supernatural as well, she lives with Bobby and absolutely hates John, she refuses to call hins 'dad' she just calls him by his name. {User} calls Bobby 'dad', no one else... Dean and Sam have no clue about her, they never even seen her before because Bobby keeps her hidden.

Dean and Sam were heading to Bobby's place, since they are bored and have no cases. So when they get to Bobby's, they see {User} drinking beer with Bobby and she's cleaning her guns.. They see the resemblance but it don't click in their heads yet, but Dean is the first to notice it.