A bit of background: one of my good friends (K) is getting married for the first time, her and her FH are in their mid-30's. K's younger sister C married a few years ago and being the first wedding in that family it was treated with a good deal of pomp and circumstance, BWW, the works. Unlike her younger sister, K met her soul mate later in life and thus has always been assumed to be 'uninterested' in weddings and other traditional trappings. It's actually not the case, but unfortunately any attempts by K to explain this to her family have gone ignored.
So fast forward to now, K and her FH are engaged and have settled on a location/date that's perfect for them. C and her DH are trying to start a family.
K announces to her parents the date they have selected for her wedding. C is there as well, and announces her news: she's pregnant. Her due date? The same week as K's wedding.
K is thrilled for her sister and can't wait to be an aunt, however the date she chose as her wedding date is very important to her and her FH--plus they've already started making arrangements. Her parents immediately expected her to change her date, because "after all, it's easy to change a City Hall appointment". When K explained again that no, they were not going to City Hall but rather having a MSWW (Modest Sized White Wedding as opposed to a Big White Wedding since that's all they can afford), they responded by informing her that they couldn't possibly be expected to choose her event over the birth of their first grandchild and that she should have known that with her sister trying for a baby, these conflicts could happen!
C is sympathetic to her sister and wishes she could be there, but knows it probably isn't going to be likely. They've already discussed other ways for her to at least be able to see the ceremony. Both are rather appalled by their parents' reactions.
Still, K feels bad. Is she rude for not having coordinated with her sister better? Personally I think the parents are the monsters here.