Hm, now that she asks that... "I'm not sure. We just talked about that guy- I think he's the family butler or somethin'? But I know she's got three kids, and she sounds close to her brother. Maybe they all live together?"

For all Tidus knows. But what's living together when you're going on adventures as often as it sounded like Della was? He really had a lot more to learn about the plucky mum!