In the worlds of bluegrass and traditional folk, where music is still passed directly from one generation to the next, it’s not unusual to witness young talents astounding crowds with feats of instrumental dexterity. But it’s highly unusual to encounter players like brother-sister duo Giri and Uma Peters, who, at ages 14 and 11, have already performed at the Kennedy Center, jammed and recorded with Rhiannon Giddens and tracked an album at Jerry Douglas’ studio — after gravitating on their own toward American roots music.