I'm with TurtleDove, Roe, Hmmmmm, Lynn, and WillyNilly.
Certainly it's not "rude" of K if she doesn't change the wedding date. And I also agree that her parents weren't nice about this (assuming we are getting a verbatim quote of what they really said) and would be out of line to demand a change. This sort of thing comes up all the time, and couples usually try to avoid times of other family members' weddings, graduations, and so forth -- not because it's "rude" not to, but because it makes it easier for everyone. (Does anyone else remember the string about a couple who wanted to get married on a day that had a date that was interesting mathematically, even though it was a non-holiday Thursday or something, and they were inviting many people from out of town? And there are couples who have personal reasons for wanting to have their weddings at sunrise on the beach, or midnight on a mountaintop. Of course, yes, they have a right to plan a wedding any way they wish. But they don't have the right to insist that no one is allowed to think that they are putting a frivolous reason ahead of all the guests' convenience and comfort.)
So although she doesn't have to, and no one should insist that she must, I don't see why K herself wouldn't want to change the date to some time when there isn't something else so huge happening in the immediate family. I understand the sentimental attachment to the date they first said "I love you." But I bet there are other significant dates on their personal calendar, and anyway, I suspect it's not THAT important to them that it be the wedding date. They can celebrate that anniversary, too.
Honestly, I think the real issue here is the way the parents seem to have treated the two sisters. It's not C's fault, and I am pleased to see that K seems to understand that and not hold it against her.
But to me the issue isn't a question of rudeness or even acquiescing to or defying parents, whether or not they are being mean or unfair.
It's about trying to make things easier for the whole family. And K herself is probably going to want to share the excitement of the new baby, too -- and for that matter, I wouldn't blame her for not wanting to risk her own wedding be upstaged by the birth of the baby just before, or expected just after, the wedding. If I were K, I'd definitely want to plan to maximize the chances that everyone could share fully in everything. It would be a shame for either sister to have to try to minimize her own wonderful occasion. And no matter how mean the parents are being about things, now or in the past, really, it's not so nice to put them in the position of possibly having to choose being with one daughter as she marries and the other as she gives birth.
Not to be a big downer here, but there is one more thing to consider. Remember, too, that unfortunately not every pregnancy and every birth goes smoothly, some moms are in medical crisis, and some newborns are in grave medical circumstances that require surgery. What an awful position K and her parents would be in then. And it would cast a pall on the wedding.