AIBU to not forgive SIL affair?

[Ambs81]

This is a complicated one- but I'll try to tell the short version! Any advice, especially from those who've experienced similar would be great.
Last year I found out a work colleague had split from their husband of 10 years after finding out he'd been having a 6 month affair with a younger women, who was known to my work colleague.
Like anyone, my work friend had completely broken down, she'd confronted the girl he was having the affair with -who she knew well and after a few days, rang the fiancee of the girl involved to make sure he also knew what had been going on.
She found out when typically, her idiot now-ex-husband had left his phone with hundreds of photos, video, messages from the other women in an archived chat. Most of it was quite sexual, but they had also told each other they loved each other, and discussed plans to leave their partners (talking solicitors for divorce, living arrrangements).
The plot thickened when a week later, it was mentioned in passing the name of the finance of the girl involved in the affair - it was my brother in laws name (my husband's brother). Its not a very common first name, or surname (now my surname), i was surprised my work friends hadn't already made the connection.
We quickly confirmed it was my BIL financee (they are due to get married this summer), I was shown some of the images / video involved and it was clear she was the guilty party, I also noticed she'd come off social media at this point.
I obviously told my husband, and explained his brother already knew what had gone on - but encouraged him to talk to his brother and support him through the fallout.
His brother was extremely upset we knew, and as his fiancee had managed to convince him the affair was just text messages, not sex, our input confirmed that it was sexual (i watched footage of them having sex!), which obviously caused my BIL more upset.
Long story short, 6 months later and my BIL has decided to stay with the his financee, after 2-3 days living apart, they decided they were both to blame and wanted to stay together.
My BIL suffers from depression, and it seems as though the family feel this caused her to have an affair, as he was distant and not meeting her needs. As someone who has suffered with depression, I feel really shocked that this is being used to justify her betrayal. Also worth noting she was my BIL first relationship/ girlfriend, so he has no other frame of reference.
They've also decided to go ahead with the wedding this year, 10 months after the affair came out. While I think it's far enough to stay together, I think going ahead with the wedding as planned is a joke - especially with it taking place in a catholic church.
While the rest of the family have totally forgiven and forgotten, my husband and I are really struggling to come to terms with it and treat her as family. My husbands family don't want it to be talked about, it is completely swept under the carpet now, but I havent directly spoken to my BIL or his fiancee since this came out (6 months ago).
At the request of my mother in law (who i greatly respect, she has helped me a lot with my DS) I did reach out to the girl involved, suggesting we meet and bury the hatchet. The reply was quite a rude and basically indicated they were angry at us, and that she was willing to accept me as family 'no matter what' (inferring I'd done something wrong??).
Since that exchange of messages there has been no contact, I'm avoiding family situations/ occasions where they'll be and visa versa.
I'm happy to continue with this, but with the wedding approaching i feel it is going to come to a head, as I don't want to attend and don't want my son to attend.
I would go out of respect for my BIL (who was a close childhood friend and how I met my husband), but as he hasn't spoken to me since this happened I feel that I have no relationship with the bride or groom, and it feels bizarre to think of going to their wedding when I'm not on speaking terms with them.
I still feel so angry at her and I feel devasted for my BIL, because he is such a lovely guy and I feel he is being duped. None of his close friends know the truth and I worry the only people he confinded in - his parents- were more worried about maintaining the status quo (as the wedding invites had gone out, they own a house together etc) then his self esteem and self worth.
As the family have moved on so quickly, I feel they think IABU to still hold a grudge and not let it go, I also know it will hurt my MIL and FIL to not have me at the wedding, more then anything because they wouldn't like their friends to think/know they was an issue/dispute in the family.
I know I'm taking the moral highground with nothing to really gain, but just can't move pass the fact that she did this to my BIL and also to my work friend - who has now ended her marriage of 10 years.
It seems wrong that on one side a marriage is over, but on the other its business as usual and the wedding planning is in full swing.
Had her cheating been a one off, or even a two-off thing, I wouldn't feel as strongly but it was such a prolonged affair, with meeting for sex 1-2 times a week in my BIL house, there was also a lot of bad mouthing of my BIL in the messages exchanged, and she was putting a lot of pressure on the man involved to get a solicitor and get a divorce.
I also find it weird that while my work friends marriage was in a bad place - they were having counselling - my BIL was unaware of any problem, and his fiancee seemed fine and very actively planning her wedding.
Another side note is my own brother, having cheated on his fiancee went ahead with his wedding and then split after 6 months - and still can't start divorce proceedings as haven't been married long enough. I wish I'd advised my brother to delay his wedding, but as he was the wrong-doer I guess I encouraged him (or at least didn't discourage hime) to make things right by going through with it (in the end it was his wife's choice to end it). I'm raising this as I have recent experience with how marrying of the back of infidelity is never a good idea!
Sorry for such a long post...I guess my question to you guys is AIBU to not forgive and forget, and AIBU to not attend wedding?

I know the thread has turned a bit of topic...but my original question was AIBU to not forgive her affair, and AIBU to not go to the wedding?

Re; AIBU to not forgive her affair
Based on general response for posters on the thread - I think I am being unreasonable, I agree that I am overinvested in the situation, have made it about me - when of course its not. I thought about another SIL I have (my husband sister) if her partner cheater, I would 100% forgive and forget. I really like her partner, so it would be easier for me to believe it was a mistake and wouldn't happen again, and I'd be keener to brush it under the carpet.
of course, knowing the 'victim' has made it harder for me to detach from the situation, but its better for my DH, DS, BIL, PIL and ME, to just step back and I will (try very hard to).

To be clear, despite how some comments have portrayed it, i havent ever been negative or nasty about the SIL or situation, I have just tried not to say anything and keep away, but I know this in itself will speak volumes.

Re; AIBU to not attend wedding
The general conses was that i shouldn't attend - but some people thought I was being unreasonable in doing this.
One suggestion was that I should go, but not bring DS.
This would be the most comfortable scenerio for me, the SIL in question is really over the top with my son, and it does make me uncomfortable. Before people jump down the neck about this, there is some background to me feeling that way (she is very over the top with my nephews, but then openly is very negative about them - even accusing a 6 year old of being sexually inappropriate towards her- in front of other family members). I think as a mum you have to trust your instincts and I don't trust her around my son, obviously nothing bad could happen in front of my eyes, but her attitude towards him makes me uncomfortable - and always has, long before the affair.

However, I am also confident that on her wedding day she is likely to not have time to fuss over a toddler, and I know I wasn't picking up kids in my wedding dress/ wedding day anyway.

So my current plan is to play it by ear, the wedding is at the end of July - if my start of July I am still not in contact with BIL - and no bridges have been there - I will probably not go.

Saying that, if my MIL and FIL do choose to raise the topic, and insist I go, I will go out of respect. And contrary to suggestion, I will not go with purse lips and be a dick all day. If I'm there i will behave like any decent wedding guest.

I few people have rasied 'concern' for my DH and suggested i have been making his life difficult with this, which simply isnt true. It is not something we discuss, unless prompted to my a phone call or comment. We both know how the other feels about the situation, I also respect why and how he must rebuild relationship with BIL- this is his priority and mine.

He is not pushing my wedding attendance because we just aren't that kind of couple, I wasn't born in the UK and subsequently most of my extended family live overseas - I frequently have to attend family functions in my home country, sometimes he attends, sometimes he doesn't. I wouldn't want him somewhere against his freewill - and he feels the same re;this.

We are not one of those 'do everything together couples', even though sometimes I'd like to be! My DH feel he HAS to be there, i 100% agree, but I don't think that extends to in laws/ extended familes. On our own wedding day his step sister cancelled due to personal reasons, I was a bit put out but it didnt overshadow our day at all.

I don't think tongues will wang if I'm not there - as some of the earliest commentors said, it could be as simple as saying me or DS is ill and thats that.

So thats where my thinking is currently!

Any further comments welcome, but please don't 'troll' me or others - and please keep relevent to questions asked if poss

very balanced decisions OP.... go with your own flow