Originally Posted by Mike Williams

But how would Jack find out? His dad is dead. Claire doesn't know. If Claire doesn't leave the island, it's not like she'll run across a picture of Jack's dad in his mantle. Surely the connection must be significant somehow, instead of just, "Ooooh, that's weird," but whatever way the come up with to let Jack know Claire is his sister better be really, really good or it will just be stupid. I think it's better that we know, but Jack doesn't.