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!