Yes. Incestuous marriages were very common for centuries, particularly in royal families, but also in small hamlets as well. In tiny communities like that almost everybody is at least a second cousin.@Cal Jeffrey Wow, with a memory like yours, who needs box sets.
The plot twist that Luke and Leia were siblings was not unexpected, perhaps even predictable. But, having a dirty mind, I always thought it was written mid-script, not at the outset.
Besides, that coupling couldn't be exposed while maintaining a PG-13 rating.
In any event, all this led me to do some research on incestuous royal relationships. Apparently, "Game of Thrones" (as well as is real life),,is full of them.
I have 7 seasons of it in DVD, 6 1/2 of which are unwatched. (I didn't buy season 8, since nobody liked it, and as I understand it, the naked girl in the blonde wig with the dragons, didn't get the iron throne. Bah, humbug.
Oh and yeah, the sibling reveal did seem kind of off the cuff to me too.