Nickelodeon’s “Dora the Explorer” drew huge ratings with an episode in which Dora’s twin sisters were born. The episode, which premiered on Monday, was the highest-rated Nick Jr. telecast in its time slot in almost three years, drawing a 10.8 rating among kids 2 to 11, or 1.3 million kids. That rating was up 72 percent. The show also drew 2.5 million total viewers, up 47 percent from last year.