I think Jin visits to give the advice if you haven't bought her at that point? hard to remember as I usually avoid story events while I get the bar up and running enough to buy everything.
But how is that logical? Jin is someone else's slave. She's not going to just drop by because Lin has a sex question. Especially because the fact that I haven't purchased her means that she's going to disappear again once they have the conversation, and Lin apparently won't care? Also, Lin and Jin very clearly have an ongoing relationship in the scene in which she appears. There's not a bit of surprise that the other is there. That casual banter only makes sense if Jin has been around.
The “visit” and subsequent training with Jin for Lin has nothing to do with or is in anyway tied to the purchase of Jin.
Then why is she there? She doesn't even know Lin's alive, if she hasn't been purchased by the MC. The "visit" involves no surprise, shock, or emotion at the presence of either sister. She just shows up and they have a conversation as if they've been having them all along. It's grossly illogical, and clearly either a programming error or a characterization error.
I would guess that in your save Jin has not appeared in Lin’s room to talk to & she probably isn’t available in the dungeon either…
Of course not, because I haven't purchased her from her previous owner. She'd have no reason to be in the hotel, or to know where it was, or to know that her sister was living there, because in that version of the story Jin is unaware that Lin is alive.
In my admittedly shoddy memory…I am fairly certain that you don’t even have an opportunity to purchase Jin until after the training visit
No, that's wrong. Jin's been available for ages. I just hadn't pulled the trigger.