The Infinity Caverns - Parallel Society, #1

There are other worlds. Other universes. And sometimes, they slip into our own. When Veronica "Roni" Rider discovers this, she also learns that there are three people with magic powers tasked to save our universe - her grandmother and two seventy-year-old friends. But it's hard to defend your world with a bad back and arthritis. Tossed into a new reality that she never expected, Roni must decide if she will step up and join the team or forever leave behind all she has learned and all she cares about.
Oh, and she'll have to save the universe, too.