In Diablo 3, when does deckard Cain discuss "Leah and Cain's travel (Act 1)"?
It becomes available after you return from the Drowned Temple with the second glowing sword shard, before you take the ferry to Wortham. The easiest way to get to this point is to start your game at "7. The Doom in Wortham". The dialog option will be immediately available from Cain at this point.
Be sure to talk to Cain before accepting the next quest from The Stranger as when you do so Cain walks into his house and then you can't talk to him anymore.
I was able to talk to him right after placing the stones in the pillars before entering into the temple, I think that that is the best time to complete this challenge