User blog comment:Ncduru/The Erza-Irene Connection/@comment-24784846-20160427172729

Plot Twist: Irene and Erza are related, but they are far from family. Erza was previously removed from her family. She stopped being a "Belserion" for some reason which is why she doesn't reveal her last name to the Tower of Heaven crew. Possible reason is that she was thrown out by her own family for being a potential disgrace to the Berselion name.

Or who knows, maybe Erza was moved to Ishgar by Irene, because her family was crazy and wanted to destroy Erza's childhood by turning her into a warrior. They had cousins in Rosemary Village, but the died during the ToH kidnappings and massacre. Potentially, Erza was told to forget she was a Belserion.