I mean there are a lot of characters who are related to alternate earths who live here now that we are just going to have to live with thanks to Crisis on Infinite Earths, and not just surviving characters but also their children like the Ray or Damage or Starman(jack knight). It just works is pretty much how you have to deal with it.
Also I got to be honest i always felt that thanks to Crisis and Infinity Inc that Powergirl was more in Dick's generation of heroes, she is more in his age group because she has got to be what mid to late 20's, possibly early 30's. I'm just saying she was around when Dick in New Teen titans became Nightwing. If anything Kara Supegirl is in Tim's generation from when she showed up and who's age she is closer to.