Well, this thread has taken me a fair bit of time and annoyance to get through. It's is absolutely CHOC full of inconsistencies, drip feeds, and tbh what sounds like made up bullshit which has been changed to suit the OP's narrative when things aren't going her way. I.e. from the start and from approx 98% of replies
Firstly, unborn child miraculously jumped from going to be 4mth old to 6mth old when it was pointed out that cruises don't take 4mth old babies.
Nephew who is already 17, is miraculously going to be still 17 in a year's time.
Nephew (who will probably be 18) is going to be allowed to be swapped out for an infant as a "child" place. (17yo and 6mth olds are not categorised in the same bracket of "children" on a cruise ship).
OP originally stated she couldn't book another room because there was no extra adult (wtf!?!), when there are eleventy billion adults sailing in their party.
Two different people who are in the cruise ship industry as well as several prior travellers have stated that it is absolutely possible to add an unborn baby as a passenger and add the details after the birth, however apparently not on the OP's special cruise line.
The amount that the OP paid towards her parents' cabin has varied wildly as the thread has continued.
The OP is pissed that her DM invites her grandson on her holiday, but also invited her own parents on the holiday that had been organised her her PIL 😂 pot kettle.
OP has in one of her most recent posts, admitted that from the moment her DM invited the nephew, she was unhappy about it - it would seem that her now being pregnant has finally given her the excuse she wanted to try to kick her poor nephew off the holiday and came to Mumsnet looking for validation that she was doing the right thing. Which, btw OP you are absolutely not.
I really hope the OP's parents pull out of the whole thing given that they're going to be put in the position of not only disappointing their grandson, but (if it's to be believed), being "kicked off at" by their son whom they are scared of. Then the OP will have no choice but to book another room - in which case the cruise line will no doubt miraculously change their policy on allowing unborn babies to be booked in advance and their details updated when they are known. 🥱