People don't put together that the two people named "Skywalker" are related?
Watching prequels first is shitty, George Lucas is an idiot. "No, I am your father" is the reveal you don't want to ruin.
I showed my now-fiancee the OT for the first time in 2015 (in the run-up to TFA), and I was surprised how much hadn't been spoiled for her by way of pop culture osmosis.
She knew about Vader being Luke's dad (because it seems to be THE spoiler), and enough jokes had been made about the incestuous undertones between Luke and Leia that I'm pretty sure she knew they were siblings, but nearly the entire plot of Jedi seemed to have been a blank slate to her. Yoda's death and Vader's sacrifice both made her cry, which was interesting to me because I think seeing the movies as a kid meant I didn't internalize the severity of those characters dying, and I just took it for granted as an adult that that's what happens in the story.
Also led to this amazing conversation before we watched any of them:
Me: "So yeah, they're getting everyone back for Force Awakens. Mark Hamill, Harrison Ford, Carrie Fisher..."
Her: "What about James Earl Jones?"
Me: "No, he's not coming back."
Her: "Doesn't he play Darth Vader?"
Me: "Yeah."
Her: "But how are they going to do a new Star Wars without Vader? Isn't he like, the bad guy of the whole series?"
Me: ... ... ... "YEAH IT'S REALLY WEIRD ISN'T IT"