Who says it's a force connection? Since when does a jedi (or trainee jedi) require a connection with a certain person in order to communicate with them through the force? Siblings or not, Luke and Leia are close. Who else was he gonna reach out to on the falcon to save him? Threepio? Nope. Lando who he's never even met? Nope. Chewie? Well, maybe, but surely Leia's the first choice. Stop making excuses for Lucas (and stop belive the excuses he's given to you)

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Just think if he called to Chewy on the Millenium Falcon instead of Leia, and Chewy heard him through the force, Han Solo was the only one who could understand him. You would have this crazy wookie howling in wookie language, "Go back, we have to save Luke! Go Back!" And Lando & Leia, not understanding a damn word he his howling, would shrug at each other, "Maybe he's really hungry or something Lando."