Alexandra and Robert Switala, a brother and sister violin-viola duo from Grapevine, will appear Sunday on From the Top, the NPR/PBS program about young musicians performing at Carnegia Hall. They’ll play Johan Halvorsen’s jumpin’ Passacaglia on a theme from Handel for violin and viola. Their playing the final variation, back to back — so they can’t get any visual cues from each other — is a bit of a feat. Both Robbie and Allie study with Jan Mark Sloman, associate principal concertmaster with the DSO, and Allie will play with the Fort Worth Symphony this fall.