Ive been together with my partner for 3 years.

Ive recently discovered he's been watching porn, which does bother me slightly but only because he hasnt been very attentive to me recently. I found it on a shared tablet that we both use.

Anyway it's not the main issue, my issue is the type of porn he's been searching for. It all seems to involve "brother" & "sister". Like actually searching those terms not just browsing and happening upon a video. I have tried talking to him but he gets defensive.

In all honesty its made me feel sick and I'm not sure if I could ever let him touch me again.

He does have a stepsister (if that is any help)

Time to take the porn out of the closet! You found your boyfriend's porn on the computer you share with him -- so on some conscious or subconscious level, he wanted you to find it so that you're in on this secret he has. If he really wanted to keep it secret, he would have worked harder to do so. This is his way of trying to share and start a conversation. Whether or not you like his style, this is what you're working with, so let's move forward! You need to bring this up with him. Tell him you found his porn -- just like that. And let him react. Don't paint him into a corner with judgment. And if he doesn't react, ask him if he wants to talk about it at all. If he doesn't, tell him you do. Ideally, you'll have a conversation and that's the time to tell him that you're disturbed by the incest in his porn. Subject matters like these are typically kept quiet -- and not discussed because they're uncomfortable and awkward. But that's the worst thing you can do, because by keeping his secret quiet, it becomes your secret that is kept quiet, too -- and then you're being complicit in enabling shame. So air it out and talk about it. You've been dating for three years, so you probably think you know everything about each other -- but many long-term relationships have secrets kept for way longer than three years, so use this opportunity, as distasteful as it may be, to get to know him by discussing this issue. If he does have an incest fetish -- or if he's been molested in the past or had some type of sexual history that is troubling -- now is the time to address this. So, before you break up or assume what may or may not be.... talk to him. This finding is a cry for help -- and an opportunity to get to know each other beyond what you thought you already knew.