The thing about that is that it is never explained how L&L knew they were siblings. When Leia announces to Han that Luke is her brother, it’s quite out of the blue. Who told her? When? Is this just another Force hand-wave?
Star Wars was a great cinematic feat for the 70s and 80s, and a testament to the power of practical effects; however, it has always been a masterclass in hack writing, audience condescension, and hamfisted acting