It occurs to me that we are all picturing such different situations.
I was baffled by posts that said that it would seem weird to discuss dates with immediate family before committing and that a mother of the bride's time commitment is only 12 hours max.
My point isn't that those posters are wrong. Not at all! But they are picturing very different scenarios than I, and it seems many other posters, are.
Some weddings are small, and everyone lives in or near the same city, and there aren't many out of town guests, so the wedding is a few-hours event, and the parents are coming in from out of town, and the bride and groom are doing all the planning, and it's a family and community in which weddings are not so family-focused as in others, and the bride and her parents don't have the best relationship. So, yeah, then, maybe, 12 hours.
But not all weddings are like that. Some weddings are family-centric gatherings 200 people, and in a city where one set of parents but NO ONE else lives, and the parents are hosting, and there are a hundred and fifty out of town guests coming for a three-day weekend, and the custom in their family and community is to entertain all of them the whole weekend. (I have just described my son's wedding a year and a half ago, and probably my daughter's wedding next year.) Twelve hours? Yeah, right! Sometimes I feel like my daughter is expecting me to focus on it 12 hours a DAY! (I'm not complaining; it's fun doing the planning together.) She absolutely checked the date with everyone before choosing it, let alone making any deposits. She even asked her brother and sister-in-law if it was okay with them if she chose Labor Day weekend, as they had.
Ditto babies. When my children were born, my mother came for a few days before the due date to grocery shop, cook, and learn how our house runs so she could run things and take care of the other visitors and let us rest and focus on the baby. And I was so glad she was there for the births, too -- even in the room the second time. She was such a huge help. The second time, I had preeclampsia, and had to lie down for a week before the birth; I don't know how we would have managed without her, especially with a preschooler to care for. She sure couldn't have left to go help someone with a wedding.
But in other families, the grandparents are the very last people the parents want around at a birth. And there is nothing wrong with simply wanting privacy.
My point is that we are all looking at a different part of the elephant, as it were. In my family, it would really seem strange not to precheck a wedding date -- but look how many posters say it is exactly the opposite in their families. And in my family, I can't imagine someone not changing a wedding date from a sister's due date week (just for the sake of it being the date of the first "I love you" -- it would be very different if there were some compelling reason that would be really hard to get around). Not because it would be seen as rude not to, but because both sisters would want very much to maximize the chances that they could be at each other's events, in addition to trying not to make it too hard on the parents. But not every family is like that, and what would work for some would not work for others at ALL.
I do think that just because it would be POSSIBLE for the parents to juggle everything, it's not unreasonable or "making excuses" to prefer to space out the events a bit.
K knows her family, she knows how much she cares if C can attend her wedding, and she knows what will work best for her and for them. And I'm sure that's what she will do!