The Final Crisis: Legion of 3 Worlds miniseries reveals that the Tornado Twins and their family members (including XS) originated on the same Earth as the post-Infinite Crisis version of the Legion (which was introduced in "The Lightning Saga").[1] Thus, continuity revisions have been made: after Barry Allen's death in Crisis on Infinite Earths, the Twins and their families became the target of Professor Zoom, who was attempting to sabotage Don Allen and Meloni Thawne's marriage. Both families escaped to Earth-247 and the Twins died soon after in unknown circumstances. Both cousins were reunited with their grandmother Iris, and both also rapidly aged to teenagers within days. Presumably, XS's rapid-aging stabilized under her own volition, as she never showed signs of it during her tenure with the Legion.
XS is used by Brainiac 5 to revive her cousin, Bart Allen, culling his spirit from the Speed Force. The two then team up against Superboy-Prime.
After the battle with Superboy-Prime and his Legion of Super-Villains, XS resigns from the Earth-247 Legion. Bart asks her to join him in the 21st Century, but she decides to stay in the 31st Century with the post-Infinite Crisis Legion to locate any of their surviving relatives. Her friend and teammate, Gates, also decides to stay to accompany her.
XS possesses Speed Force-based powers like the other members of her family (Barry Allen, Bart Allen, Wally West, and Iris West). She can run at near-light speeds, vibrate through solid matter, and even time travel and cross dimensions by altering her molecular vibrational frequencies.
XS made a cameo in the Legion of Super Heroes episode "Dark Victory" Pt. 1.