Star Wars facts to impress your nerdy friends!

I mean, we know that she knew. In the ceremony to acknowledge her as heir to the throne of Alderaan she used phrases specifically for adoptive children (Princess of Alderaan by Claudia Gray), plus she looks remarkably unlike Jimmy Smits.

However, it wasn't super publically talked about. In Alderaan, your adoptive parents were your parents. People didn't care who physically made you, it mattered who raised you. It's also a weird thing to bring up right after your parents have died.

All Luke knew about her was that she was a princess, which would typically indicate that she knew who her parents were. One of them had to have been alive up until...

In fact it's unlikely Leia didn't at least mention the fact that both of her parents had just died. Luke was probably fully aware that she knew the identities of both of her parents, and he probably had no idea that they weren't her biological parents (again, weird thing for her to specifically bring up right after they've exploded).

I know there was some Force Stuff™ at play here but idk it's just a weird leap for Luke to have made.

"You have a sister who is also the secret child of Darth Vader", "oh, it must be Leia, who almost definitely knows who both of her parents were."

Which brings me to the conclusion that this was Luke's entire thought process

they accidentally had c3p0 say “die jedi dogs” in the clone wars movie but forgot dogs dont exist in star wars so they had to invent dogs

Many of the elaborate background “sets” from the original Star Wars trilogy were actually just detailed oil paintings on panes of glass. Source Source 2 Source 3