Sorry @familydrama1 I have only read all of your posts, and having never been on a cruise, or had any problems like yours, my suggestions will probably br silly, but here goes just in case ...
Who's names are the different rooms booked in? If your room is booked in your name can't your husband book a room in his name, and have one or both of your older children in with him, and vice versa if your cabin is booked in his name? I know that would mean you and your DH couldn't sleep together, but while sharing with children in your room I don't suppose you could, or would even want to, get intimate with your DH overnight - but you may be able to sneak some time together during the day sometimes, as you will have a lot of babysitters available by the sound of it!
If that wouldn't work, could at least one of your nephew's parents come along and book a new room, then have their son - your nephew - in with them? Therefore that would leave your parent's enough room to have one of your older children in with them, while your other child, and the baby stay with you and your husband in your cabin?
If there really is no feasable way for you all to be together, then I think you should let everyone know that sadly, due to the joyous news of your precious pregancy, you can no longer go on the cruise. I would then leave it a few days before actually cancelling your part of the trip (as someone else might come up with a different scenario that no-one else has thought of yet).
Dear OP, if you do have to cancel your holiday, I hope that you bought travel insurance for the four of you at the same time as booking your cruise? If you did do so, the insurance, unless very basic, will surely cover you cancelling since you have unexpectedly fallen pregnant?