OMG, good luck today!  I'm also in the UK.  One of my SDs marrying, is one of the things I dread, so I can well understand your feelings. It's hard when the degree of animosity doesn't decrease over such a long time.  Keep those body cams on! 

Hoping for your best possible outcome today.  Let us know how it goes.

Body cameras--what a great idea. Wouldn't it be fun to be able to share the images here? 

You know we are waiting to hear how everything is going/went! Our SD wedding "experience" was awful. Hoping yours is much better! 

Thanks to all of you,  your messages were very much appreciated, and the wedding went as well as it could have done.
A very brief history, and by no means everything, DH left BM  21 years ago after attempting marriage guidance sessions, during which a strong matriarchal line of abuse was recognised on BM's side. BM was urged to recognise this abuse,   and to try to break the cycle. At this point both BM and DH had been estranged from her family for a good number of years,  this didn't change until after he left her.
When we met  and fell in love,  the hell and damnation ensued. We had a good 2 years of constant harassment and severe behaviour including constant false allegations to the police.  This was eventually recognised and rectified with a formal police caution.
Obviously the SD's were used and abused during this period.  OSD lived with us right from the beginning so the effect of this abuse was nowhere near as bad as it was for YSD, but she still has lasting issues. Our relationship with OSD is good.
YSD moved in with us 2 years later after being beaten up and abandoned in the middle of nowhere by BM for refusing to make  fake allegations of sexual abuse by her father. There was no contact between BM and YSD, on BM's instigation, for around 6 years  from the age of 12 to nearly 19. BM also physically assaulted me 20 years ago.  YSD had just been expelled from school for extreme bullying (indicative) and we worked hard, firstly ensuring that she was accepted into a mainstream school and then getting her through the estrangement with her mum. She went through university successfully, and has a decent career.
10 years ago aged 18/19 she changed and became increasingly hostile, and more and more aggressive with me,  to the point where she had to be physically restrained on a number of occasions. This behaviour eventually spilled over towards her friends and ended very badly for her. Luckily for me, the big event that caused chaotic disaster  for her, occurred whilst we were away on holiday,  and luckily for her, 2 weeks before she went away to university, where she was able to rebuild her reputation and make new friends. She also re-established her relationship with BM.
We rebuilt our relationship with YSD, and I was more than eager for this to work.  How f#cking dumb was I?
Turns out extremely dumb. 5 years ago, she restarted her campaign against me, and 3 years ago we disengaged after she used her pregnancy and birth of her 1st child against me and then against DH because he didn't comply with her wishes and dump me 😅🤣😂(she's currently expecting her 2nd)
Back to the wedding,  YSD and BM attempted to hijack, but ultimately it all went horribly wrong for them, YSD forced OSD to allow BM to attend the hen-do (well documented) and BM pretty much ruined it.  Her final tantrum  occurred in the departure lounge of a spanish airport,  ironically against YSD,  at top volume, and incredibly nasty.  It reduced a number of the attendees to tears. Nobody spoke to BM including OSD until the wedding.
Both SD's obviously have issues,  YSD completely covers for BM's behaviour, OSD doesn't,  but has always felt a responsibility for her sister because of her mother's behaviour, and so she excuses and covers YSD's behaviour.
Both partners of SD's intensely dislike BM.
No-one can reason with BM, it makes her so much worse and causes severe eruptions. She's clearly unwell.
OSD wanted to bar BM from the wedding but was persuaded to allow her to attend by YSD, probably so that she had someone to talk to and sit with, because all the friends of OSD are fed up with YSD's behaviour.
I'm sorry about how disjointed and convoluted this post is, but so is our relationship with YSD 🤷‍♀️
Anyway,  the wedding was beautiful,  BM looked as ugly on the outside as she is inside,  and was avoided by pretty much everyone, YSD was civil, (surprisingly) so she is probably going to attempt reconciliation. Unfortunately,  after a lengthy smear campaign and character assassination against me, amongst other things,  this is never going to happen. My Soulmate DH feels exactly the same.
Karma is such a big beautiful biatch. And now so am I 🤣. Thank you all for your support 🙏 ❤️

I echo what JRI said, you have indeed been through some awful times.  Hopefully mainly in the past, as you and DH are presenting a united front and strong boundaries. I'm really glad the wedding went well.