Yes, when the camera is on them the sister looks as though she is muttering under her breath, over and over. The brother is doing it too, but not as clearly. I wonder if they are saying prayers throughout the hearings.
Or maybe something like "Just tell the truth, Oscar, just tell the truth" (of course I'm sure they think Oscar's intruder story is the truth.
They probably think he is veering from "the truth" b/c he's nervous or confused or emotional.
But actually he sounds like he's lying because he is lying.