Pedro was her brother. I don't recall anything I identified as particularly incestuous about them--if anything, I thought the subtext was that the doctor was in love with Mercedes.

I only caught a lot of the parallels in the film in retrospect. While I was watching it, I was so involved with the story that I didn't pay a lot of attention to the underlying themes and motifs, which is actually quite unusual for me. Great connection to "The Gift," too!