In 4x23, Silas told Stefan that when someone becomes immortal, nature creates a "shadow self", a "doppelganger", so, whose doppelgangers are Elena and Katherine?

I'm not exactly sure about this because I don''t have the doppelganger stuff fresh in my mind, but I think it goes like this:
Magic needs a balance to all spells because there needs to be a balance in nature. So when Silas became immortal, the balance for that new immortal creature was the creation of a doppelganger, someone that, unlike Silas, could die.
Now, Elena and Katherine are doppelgangers of Tatia but it's not because their ancestor became immortal. In fact, the original Petrova was killed so her blood could be used by Esther to turn her children into vampires.
My point is that Katherine and Elena are doppelgangers, not because a direct Petrova ancestor became immortal, but because Petrova blood was used to make people immortal.

I hope I was able to make myself clear! It's quite twisted...

They are Taita's the Petrova doppleanger's.