Your dilemma is real and your desires to make things better are well-placed. I should just leave it at that. But... Okay, here is where I decide to put something out there that will probably have a dozen posters jumping all over me within a day. So even if I'm wrong, here's at least another perspective FYI.
1. Don't EVER assume you understand the inside of anyone else's marriage. You suggest your mother would be shocked, but you could be wrong. (After all, if it was so easy for you to find.... And you only had the PC to look at; she's been sleeping with the dude and might have some other clues.) What she chooses to do with that info is probably not for her son to know.
2. Getting into the inside of your PARENT's marriage, specifically, is rarely a good idea. Confronting one parent (the "innocent" one OR the "offending" one) about the activities of the other just doesn't seem like the kovodike thing to do. I think you've chosen to stay away from this, but I want to put in my own strong vote for not being the one to rock any boats. (If you think your siblings are at risk of terrible injury, you've got a different cheshbon going.)
3. As the others said, some subtle efforts to get a filter put on might be helpful. In fact, even if the suggestions are vetoed by abba, that might help imma wake up just a little more.
4. Here's where the flamemail will start (sorry Elya), but I'm not so positive that your leaving GYE URLs near the computer are any less of a nisayon to your siblings compared to putting out URLs to actual shmutz. If they're not involved, I see little point in leaving that within their field of vision. If you want your parents to see them, I like the suggestions made of using anonymous email accounts or their Rav or whatever.
5. As much as I like the thought of saying, "wow, I heard in this great shiur that Rav XX says filters are essential," I can tell you that if my own kid came home with that I'd be a little upset that he's being so meshugene frum and one-upping his parents in frumkeit. Sorta like telling a parent that a particular kashrus kechsher is treif. Even if the info is true, I tend to resist when the source is my own kid.
6. If you listen to a word I've just mentioned before waiting for the opposing viewpoints, you're a fool....
(Has it crossed your mind to just install a K9 filter on your own, show your parents how easy it is and how much your Rav suggested it be on EVERY computer around you including even when you visit your parents? [They DO want their son to visit, and to feel comfortable, right?] You could of course offer to UN-install it, but how could abba request THAT with a straight face! The suggestion earlier of setting the password, giving it to them [so they can protect the other siblings -- of course] and then just changing the password the following week... BRILLIANT!)