Neither "en datter og en sønn" nor "en sønn og en datter" sound off to my ears, and I'm getting a generous amount of hits for both word orders.
Here in Norway, I think people would tend to mention their firstborn first, regardless of gender. It wouldn't surprise me if there were a stronger tendency toward mentioning the son first in older texts though.
I feel like "aunt and uncle" sounds way better than "uncle and aunt." "Daughter and a son" doesn't feel that off to me, but "son and daughter" rolls off a bit smoother. "Brother and a sister" seems to me a lot more natural than "sister and a brother." "Nephew and niece" is also preferred.