If she gets you one on one (or even in a roomful of people) just tell her "That's good well good luck with it all" or something simple like that. As stated just be polite. If anyone (most likely will be your cousin) asks on your opinion there about being preggers in general there's' no harm in honestly saying you don't get the big deal about it, that yeah another person will be born but overall not too wrapped into the hype over it and just hope your cousin's goes good for her. I get it's not easy to say things when you can't be excited over it or even want much to do with it, but as stated since it's a big event to her just being polite can go a long way. Also it's better to hope it goes well over bad as the aftermath of losing it can do unrepairable emotional/ mental damages. Not all the time but many times it does and if she does have a happy life so far with her partner losing the pregnancy could be worse than any fight you've seen them have.

Also do busy yourself with things where you're going if possible. Offer help in getting things set up, store runs for last minute things if you have a car open to use. Otherwise I'm sure you can use your phone, bring a book or some other means of using your time to reduce conversation?

lol I feel like it's pretty obvious the op just wanted us to validate their opinions on pregnancy/their cousin but didn't want to say it outright so we got this 'give me advice' thread.

I wouldn't say cold hearted. Just self-absorbed and over dramatic.

As a pregnant person myself: Just say congrats and keep personal opinions to yourself. Or hey, don't even say congrats! Just seriously keep the personal opinion to yourself. I think that's the most important part honestly. Personally I don't even expect people to congratulate me, so if someone didn't I wouldn't even notice tbh.