Esther used Tatia-the original Petrova's- blood to turn her children into vampires, and was mentioned to have taken Tatia away to stop the fighting between Klaus and Elijah. What if she used the same method that was used to make Silas truly immortal? Nature would have corrected it, making doppelgangers to replace Tatia. AKA, Katherine and Elena the same way it corrected Silas's immortality through Stefan.

But Esther realized it was too strong, leaving no weaknesses in case she regretted it later- which, as we know, she did. So she took Tatia's blood. A vampire created through a means other than death. This would have made the Original Family a new bloodline by virtue of their death. 

The immortal Tatia escaped. Esther weakened the blood with a spell and used it to convert her children. And doppelganger creation has been mentioned as a way to correct imbalances of nature. What if the Original family creation WASN'T that imbalance?