NTA. It's not your responsibility as an older sibling. Do older siblings conceive their younger siblings and then give birth? No? It's the parents' responsibility. People with no say whatsoever in making a new human are not responsible for that human.
If I were you, I'd strongly encourage them to get him his own seat. It's much safer. With that said, you're not an asshole just saying where they are.
The people you talk to are strangers. It doesn't matter if they think your parents are jerks, because their opinion has no consequences at all. You and your parents will never, ever see them again.
You can just say, "They're in different seats," if you really don't want to say they're in first class.