Dune, I agree with what has been said - this must be reported to the authorities without delay. Ideally by the victim herself or her sister, if possible.
I have some experience in dealing with child abuse (I was a cop for 30 years). My first thought is that it would be uncommon for the father to have abused only the younger daughter.
I wonder if the elder daughter brought up the 'watching porn' topic with you in the first place as a way to introduce the abuse, or even that she was trying to pluck up the courage to tell you that she had been abused herself? That would be consistent with cases I have experience of.