Star Wars - the terrible secret finaly revealed...
Why didn't Obi-Wan tell Luke he had a sister? To protect her from Vader? Hogwash… The real reason is at the root of why Yoda looks the way he does and we haven't been told that he's from a race of similar creatures.
The Jedi expected some bad craziness was going to come down with the emperor and Vader - so they resorted to an ancient Jedi technique to breed a Jedi master super-soldier to defeat them. They kept Luke and Leia unaware of each other and then - much later when they were grown to breeding age Yoda contrived to have R2D2 - a walking incubator - lead Leia to Luke. They had hoped that the two would (overcome with the impact of the force swirling in their blood) fall in "love" and breed to produce a powerful inbred banjo-playing super-jedi-baby.
This is something the Jedi have done for centuries - inbreeding amongst powerful Jedi families to keep the force "pure" - it's why Yoda - despite being HUMAN looks like a scrotum!
They hadn't reckoned with Han showing up and Leia getting all conflicted… you know the rest.
Remember - you heard it here first.