OP, who actually booked the cabins? Are they one booking that you made or two separate ones - one by you and one by your DP (I'm leaving In laws out of this altogether, at the moment).
If it's one, how did your DP 'add' your nephew in the first place. If it's two, how are you working the payments?
It's going to come down to who has paid for what.
You are assumedly paying the whole of your cabin and all drinks packages etc for you, DH and the two kids in your room.
You are also assumedly covering packages and extras for your DD in the 2nd cabin.
Your DP are covering their own packages, and now have offered to 'pay for' your DN to join - presumably covering his add-ons.
Whether you are 'reasonable' (and this is not the same thing as 'decent' or 'kind' in this scenario) comes down to this:
What part of the cost of their cabin (once it's configured for four) have you paid for?
If your DP paid a half and you have paid half of a cabin cost, and there is now no extra 'cabin cost' for your DN, then, arguably, the fourth bunk wasn't theirs to offer without asking as you have/had already paid for it. You haven't agreed to fund any part of your nephew having a holiday, and either your DP or your DB should be paying an extra 1/4 share back to you of he does come. You might in this scenario have a technical argument for saying he now can't come, as you need the 2nd bunk you've paid for. (This is entirely separate to whether your DP agree to share with two young kids rather than one, but we can cross that later)
If, however, your parents have paid 'half' and you paid 'half' of a 3/4 or lower cost in the first place, and there's now an extra 'cabin' cost to adding your DN, which they have paid, then you don't have any entitlement to that bunk and you never did. You hadn't bought it, no-one did - until they chose to and they can now do what they want with it. You may still be due a recalculation with the exact figures, because if you paid 'half' before you are likely subsidising your DP's 'cabin costs' a bit but I'd let that go for the inconvenience of your DP having your DD.
Which is it, OP?
I'd also like to know how long ago your DN was offered his place - a week or so, fine, if upsetting, several months, not really okay - and exactly how long before your sailing date your due date is, because it really might not be as simple to switch one of them for the other as you are making out - as others have said, cruise lines limit age mixes of children for a lot of very good reasons including evacuation safety and if your baby is even 1 day under 6 months of age (and you can't lie, you'll have to show their passport) they won't be permitted on board full stop.
FWIW, I, personally, wouldn't get on a cruise ship with your configuration of adults and young kids at all, solely from a 'disaster' perspective. It's admittedly vanishingly unlikely that you'll need to abandon ship, but if you do, you and your DH have too many kids who'll need too much help to manage this easily.