The Bloom sisters were two girls who knew Jean-Luc Picard and Louis since their youth.
Against Picard's advice, Louis went on a cycling tour with the two sisters. Though he broke his leg on the tour, he got to know them well and subsequently married both in the years to come. One of them was still married to him in 2367; she was very proud of his involvement with the New Atlantis Project and, according to Marie Picard, was "constantly reminding anyone who'll listen" of his having been promoted to supervisor. (TNG: "Family")
This character was only mentioned in dialogue.
The Pocket TNG novel, Starfall, gives the sisters' names as Kimberly and Melissa, and expands upon their backgrounds slightly. It established that they were identical twins from Australia.