Hank "Rusty" Truman asked: How would their parents and Uncle Iroh react to the news that Zuko and Azula were expecting their first child? Canon and Modern AU. How would they tell them?
Knowing these two, it'd be a big, dramatic, but still cute announcement. Lots of hand-holding and heart eyes.
The only way Ozai wouldn't be disappointed in Azula for being pregnant with Zuko's child would be if he had arranged their marriage as a way to make sure all the political power stays in their family. In any other scenario, be it in canon or on a modern AU. How dare be weak enough to care about that moron, fall for him and have his stupid kids.
Ursa would only be truly be 100% okay with it if incest was sort of expected of royalty in the canon-verse, otherwise, she'd only "accept" their relationship because she knows there's no way of breaking them up, and that if there was it'd make them miserable. So, the pregnancy would be sort of bittersweet news to her. If she wasn't aware they were in a relationship until the announcement, she might have a stroke.
Iroh would be exactly the same as Ursa... except he would then be 100% team incest anyway the second he saw his "grandkid"