Except that, sibling-angle aside, who else in the Falcon was Luke supposed to contact telepathically? Lando? He'd never met him. Chewie? Maybe, but they never really seemed all that close--certainly not as close as Luke and Leia. One of the droids? Well that wouldn't really work very well, since the Force is generated by living beings; force-chatting with a droid would have seemed remarkably out of place.

So I'm not sure this argument works. Leia was really the only logical choice for Luke to contact, sibling or no sibling.