2989 - Jenni Ognats is born the grandaughter of Iris Allen, and the daughter of Dawn Allen. She has no apparent connection to the speed force.
2997 - 8-year-old Jenni meets Legion of Super Heroes when Brainiac saves Bart Allen's life by sending him back in time.
3003 - 14-year-old Jenni is captured by the Dominators on Earth. Their experiments unlock her latent speed force powers.
3005 - 16-year-old Jenni assists the newly reformed Legion of Super Heroes in defeating the Dominators & saving her family.
3006 - 17-year-old Jenni joins the Legion of Super Heroes in their first membership drive after they reform as XS.
Jenni isn't a part of every version of the Legion. She's not a regular team member and really only shows up in the "threeboot" timeline, but she holds such a fun place in the history of one of DC's biggest superhero legacies that we absolutely had to make sure she was a part of our timeline. Jenni is a full-fledged member of the Allen family, and part of the legacy of the Flash.
The fact that Iris West-Allen survived to live in the 30th century is a regular part of the pre-crisis timeline, and there are quite a few chacters that are loosely associated with that fact... perhaps most notably the original Tornado Twins. Don & Dawn Allen were the twin children of Barry & Iris, and who actually have elements of a heroic career in a few different future timelines. These story elements never really took off THAT MUCH, but they led to the invention of beloved character Bart Allen, so if you're going to have the current Kid Flash, then you're going to have SOME version of the Tornado Twins, so you're missing a massive opportunity if you don't include Bart's cousin, Jenni.
We decided not to bother giving the twins speed-force powers so that Bart being born with his intensely accellerated and uncontrolled speed force powers would shock the family, so it stands to reason that XS would be born completely without her powers. This of course allows her to be empowered when she's made a victim of the Dominators, making her a prime candidate to be part of the newly reformed Legion.