From Bulbapedia, the community-driven Pokémon encyclopedia.
Barbara (Japanese: イバラ Ibara) is a character of the day who appeared in Queen of the Serpentine!.
Barbara can be an intimidating young woman. She became angered by Brock, May, and Max standing outside the Battle Pike. She was also angered when Brock showed feelings for Lucy. She frequently told Brock to "Shut up". However, she will bow down in respect only to her master, Lucy. Usually Lucy is the one that needs to tell her to watch her temper.
Barbara is also overly confident in her master's skills as a Pokémon Trainer as well as her Pokémon. She belittled Jessie's Seviper saying that Lucy's Seviper was way better than hers.
When Ash defeated Lucy and won his Frontier Symbol, Barbara was disappointed that Lucy didn't win.
The dub states Barbara to be Lucy's younger sister, which is probably the misinterpretation of お姉さん Oneesan (the way Lucy's apprentices refer to her), which literally mean "elder sister" but is also a polite way to refer to any older young female.