I think she would be called Mary Sue

well, they did say they will explore the history of the salavatore family. so its possible the boys had a older who died along with their mother only to find out their big sis didn't die but was turn into a angel by the celestial court and that their dad give her up to them for mysterious reasons and that it is reveal that she is silas's lover reborn.

The mother died of complications after the birth of the youngest son, Stefan. His father blamed him for her death. So an older sister who died with the mother would not make sense. And of course the mother could not have died before the younger sons were born!

The only possibility is if Salvatore father had yet another illegitimate child - but they never explained how his illegitimate son from one of the maids inherited the estate - but that is still another matter!

there could be 2 ways that damon and stefan had a older sister if she was from their dad's first marriage before he married their mom and mysteriously dissapeared or she could've been damon's twin who everyone thought died at birth but the dad secretly gave her away to another family because he didn't want a female heir.

Stefan was his father favorite son and Damon blame him for his mother death. Maybe their mother had another child from the first marriage with someone else. I already made a fanficiton of their mother who got married to Elijah and had daughter who was hybird of an angel and a vampire. I always thought their mother was a angel or witch