After all, it's a comment before I met you. Is there a real affair between them? Now that you have dug up this matter, make it clear to your boyfriend and ask him to stop paying attention to the little sister in the same city. This is a warning to him to pay attention in the future.
If you still can't get over this hurdle, you can think it over. Although your boyfriend expressed his love for that little sister, your last girlfriend was you. You're the winner. There's nothing to care about.