The self-sabotage, Marcia, when she was in the best possible circumstances at this nursing home/assisted living facility, packed up... . back to where things are 'iffy" with your sister's involvement... .
Just remember, you didn't break her, you can't fix her.
I don't mean that to sound cold... . but just how much of our lives do we have to basically sacrifice as the years continue to stack up?
Some thoughts - it wasn't my uBPD'd mother and uNpd'd father I dreaded coming to my daughter's wedding 3 years ago but the enmeshed blaming shaming sibling. And there was a church member, so like my mother, that I had helped and cared for and when times got shaky medically for me - she bailed and she was old enough to know better - one of her favorite sayings was, "I was born at night but not last night," when she would complain about a DIL I learned was probably doomed the day she married this woman's only son.
The church member was invited - my daughter said she felt obligated as the woman had made inroads into my future SIL's family if you see what I mean... . and I could really honestly expect a drastic scene with a fainty spell (the grapevine in these small communities always keeps you abreast even when you try to duck and run) and ambulance and everything as she'd had one only the week before the nuptials -
She never showed. She never came. I had to tell myself that if my FOO didn't come, and she didn't create a scene, there must be a god in heaven

.
But she wasn't my abusive parent. So this is different from what you're rightly very fearful of... .
At the same time... . what would be worst that could happen? If your mother like my late mother, the wedding would come to a screeching halt, EMS arrives etc... .
that would be horrifically typical. I wonder how many might see her as grandstanding/stealing spotlight if she took it that far?
And this is when I would get so bitter and resentful, that a lovely event would be marred and possibly ruined, time and again, by a very strategically planned maneuver.
You are rightly worried.
I wish your time before the wedding could be filled with the more usual details instead of this... .
Usually people recommend having a "babysitter" at an event for the uBPD'd individual... . someone to usher them out if it comes to a scene... .
And to answer your last coupla questions, hell no you're not terrible for feeling like this - this has been YOUR LIFE, why wouldn't you dread her behavior when you *know* how bad it can get? - and enough IS enough.
This is an event in your family's timeline that ought to be joyful, happily stressful (will the flowers arrive on time, who will lock up the church after, where did I leave that string of pearls I wanted to wear, etc

).
This is NOT the time for your mother to "test" your sister. But you cannot control her absurd sick plot or manipulative punitive stuff.
Oh lord... . my best advice is keep a sip or two of champagne on hand the entire day... . then fill the glass and toast yourself - you raised a son who has found a partner and is on his way to a family of his own.
I am sorry you are in this bind. Don't tangle yourself in it if you can avoid it.
"Normal" families - they have no idea. :P