And those tapes belonged to Katie before they got stolen, right? Isn't it kind of *beep* up that she would have a tape of her parents smoking dope and about to have sex? I guess it's a good thing that she never watched them.
As another poster pointed out, the tapes were stored at her grandmother's house. We don't know how long Katie had them. We do know that Kristi and Dan had the tapes in their basement for a year without watching them. However, given what we heard in PA2 about their memories of their childhood, I doubt that either sister felt much desire to relive the period via VHS.
Secondly, Dennis was not Katie's and Kristi's father. He was their mother's boyfriend. While it might have been "beeped" up to see her mother sort of mildly making out and preparing to have sex with a boyfriend, it's not the same as seeing her parents in flagrante delicto. As for Mom smoking weed ... Why would Katie care? Her mother was a 70's generation girl; it could hardly have been surprising that she might have smoked the occasional joint (and Julie's comments make it clear she's not a regular user).