Suddenly a weird thought visited my mind...
As we remember, Elena was adopted and that's kinda why they found out she was a Doppelganger. She doesn't have blood-siblings, only his adoptive brother/blood-cousin.
...But what about Stefan? He has a brother - who, as we think, is his true brother. But now we see, that he is a Doppelganger too. But how did this happen? Was he adopted too, in 1847, or whatever? Or did Silas has a brother? Does that make Damon another Doppelganger, or whatever?
Can a Doppelganger have a normal sibling? Or does he/she have to be like the first one? Without any siblings, or with some siblings, as the first one had?
What do you think, guys?