To provide a bit of background--yes, there is a history of the parents putting C before K when they were growing up. This toxic behavior on the parents part is not entirely new, so K had never entirely planned on having her parents play a large (or any) part in her wedding. C had been quite the brat when she was younger, but outgrew it and has grown much closer to K while realizing just how awful her parents had been (it's easy to miss when it's not directed at you).
C assured K that she shouldn't move her date out, as she put it "I didn't plan my wedding date around your life, I can't ask you to do the same". If she can make it, she'll be there, if not she asked that K just save her a piece of wedding cake.
The parents, however, have still been difficult. K received an email from them stating that if she still insisted on going through with this "wedding" (quotation marks from them), to not expect them to be excited about it, to help out in any way, or even attend.
Now, once upon a time, K would have caved in to this and done exactly what they wanted her to, regardless of what she truly wanted. Luckily, in the past decade she's developed a considerably more significant spine. Her response?
"Well, that's unfortunate but ultimately up to you. I will still extend you an invitation in the event you change your mind, as we are not changing our plans. That said, if you only want to spend the whole day complaining about how ridiculous the whole event is while making cracks about how lucky I was to find someone in my "old age", then it would be in the best interest of all involved if you stay home because you will be asked to leave if you cannot be polite to myself or DH. We are in the process of setting up a live stream that will allow C to watch the wedding online if she cannot be there and I will send you the link as well."
C is pretty proud of her big sis for standing up for herself. She's also seriously reconsidering whether or not she wants her parents there at the hospital.