I wouldn't tell her father about the rape and incest porn, because that's exactly what it is: porn. It's allowing people to fantasize about situations that they find erotic, without having to act it out. Nearly all of that type of porn is comprised of fake situations. I really don't see what the big deal would even be. Lots of kids masturbate to dirty magazines they find or watch porn. Having sexual fantasies isn't something you should shame people for. However, if you witness her actually doing these things, tell someone.
BTW, I really doubt that the people in those videos were 11 years old, unless she was looking at 4chan. Child pornographers are much too discreet to let things like that out for everyone to see. There are niche markets for of-age girls who look incredibly young. Accusing your cousin of something like that could make life really hard for her if the authorities got involved.