Wow Brendan753, I'm sorry.I feel for you and your family. My family has a member who has this problem too. We found out because the computer would 'mysteriously' get computer viruses that for a while the pc expert in the family would fix because she believed they were accidental (the first 25 times)....until she said no more and kept talking about how totally insulting it is to be asked to fix the pc stds from the sites her husband looked at. At the beginning he would be all innocent and then defensive and then argumentative, but now he has come to accept that he has a problem and is trying to overcome it
Your mom must be on board with you in order for this to stop and your dad gets help. Until she is I don't see what you can do except keep the kids away. It may be that your mom will miss the grandkids enough to do something outside of her comfort zone. Pray and fast for her and your dad. Have you talked to your sister about this problem? May be she can share with your mom some things she's observed that will help.