simple sexual harasment and blackmail both have prison sentences
Yeah, well. Proving that your daughters are sexually harassing you would need a bit more work than "she said stuff" or "she paraded in front of me almost naked" without any proof.
Also, even if we forget the fact that this is a game and not any real-life situation (as in, you know, tons of crazy, unbelievable stuff happens in AVN's), things would probably need to escalate way more than that for a loving father (which Frank is supposed to be) to decide to try and get his daughters sent to prison. Getting them to see a shrink and/or finding them their own place to live (far away from you) would probably feel like more credible, less extreme choices (I mean, if you're so gung-ho, at what point do you start hiring a hitman to take them out because they've shown you some flesh?).