Look at everything she has faced at her age... I just hate it when she is called a pick me and a selfish person... What did she not do for everyone ...
katherine's sisters died before having children. It was Katherine's baby, Nadia, she had a child before becoming a vampire and that child continued the Petrova bloodline
No, Katherines sister died before having children. She died when Klaus slaughtered Katherines whole family. But Katherine had a child, Nadia. We met Nadia in season 5 and Nadia must have had a child before becoming a vampire and that child continued the petrova bloodline, up until Elena was born and then Elena continued the Petrova bloodline when she (spoiler) had a Stefanie with Damon.
Well Katherine is from Bulgaria, so Elena has ancestors from there, since all doppelgangers are related. But Elena comes from American family, so she's American. As for Nina dobrev, she is half Bulgarian, as far as I remember, that's why Katherine is from Bulgaria. Writers could pick any country, so they chose one related to the actress
