After seeing all 150 (Pokemon diamond) and after upgrading the Pokédex to national Pokédex, Luke will say about his ‘kid sister’
His sister lives in their house in Sandgem Town. She can tell you about outbreaks. I don't think Lucas ever says his sister is in Hearthome City. You might have confused her with Bebe, who lives in Hearthome and can give you a free Eevee.