It’s an interesting one. Someone mentioned if someone they knew or someone they did know had an OF, they and their female friends would/have had a look, presumably out of curiosity. What would we think if it was a group of dads/ male friends?
I think this is a very different scenario to the PP who mentioned her ex watching her sister in the shower - as presumably that was non-consenting voyeurism and is a crime I believe, certainly if recorded.
If OP’s sister has mentioned she has this website, then it’s presumably common knowledge and accessible to everyone. Including her partner’s friends, her kids, their friends, their teachers, their swimming coaches, scout leaders, bloke at the off license, PTA, friends of siblings, friends of her parents - everyone she knows or everyone everyone she knows knows.
Should her sister’s husband have accessed the site - no. Is it unjustifiable? Probably yes. Was it an unforseen eventuality? Probably not. As someone has mentioned before, humans are curious. If people are willing to check out someone they know with their friends it’s as natural for a man to check out the site of someone he knows ho happens to be his wife’s sister. It could be interpreted as creepy AF and below the belt in terms of loyalty to his wife, but if they have a consensual agreement about the accessing of porn in general this is where boundaries come in. Would it be as bad if it was her best friend? An acquaintance? Someone she worked with? Someone he worked with? The local barmaid? Someone they only knew in passing from their village? It is absolutely horrendous for OP bc of the familial connection to her, obviously it is horrendous. Presumably her sister isn’t going to be outraged as she has told them of this site and there will be all sorts of ppl she knows able to access it - it’s hardly a secret by the sounds of it. Up to her.
The OP’s husband doesn’t have the same connection to the SIL in terms of being a blood relative so he would probably view it as no different to looking up a friend of his wife, (again out of order) or someone they both knew from their community. It isn’t HIS sister, so the grossness is not there and it’s silly to make out it is. They’re presumably a similar age and she’s not part of his immediate family. The fact she’s let them know such a page exists will have provided the same frisson as accessing porn in the early days, which for someone who has free access to it would provide something a bit more exciting perhaps as it’s slightly taboo without being illegal.
Is this cheating? That’s up to the OP to decide. Was there a conversation when your DS announced she had made this website? Did you tell your DH after she had told you? Did you tell him or imply it was strictly off limits so he breached that trust and agreement or did you assume he wouldn’t have the desire to access it out of respect for you or bc he looked on your sister as a sister of his own, so it would be like incest?
Is this out of order? Arguably yes as most ppl would assume their partners have a basic level of respect for them and would avoid scenarios such as these in the first place. But there seems to be such a relaxed and acceptable attitude to porn in our society that the lines are well and truly blurred.
If OP’s husband had accessed her site once - you could argue that this was forgivable as he was led there by curiosity, but it sounds like he went back multiple times, indicating that he liked what he saw - which creates an issue as this is someone in his family circle who he will be interacting with on a regular or semi-regular basis and it’s likely he finds her, at the very least, sexually attractive. This places OP in a terrible position, as though I’m sure there are many husbands who find their SIL attractive, (and vice versa) it’s usually discrete and kept firmly under the radar.
Im waffling now bc I’ve had little sleep. I’m sure a lot of ppl will be pissed off with what I’ve written - I’m not saying her husband was justified in doing it, but I guess I’m saying it’s not out of this world that he has done it. The fact he’s gone back multiple times is the issue and it’s someone so close to them. If it was a random woman would it be a massive issue? Perhaps yes as it shows he’s forming certain attachment, but would be unlikely to do anything about.
Does your sister know, OP, and how did the topic come up in the first place?