Nanoha gave a concerned look to Saber before smiling at Hayate. "Uh-huh! My mom and dad gave me a grocery list, so we're doing the errands. It's a bit of a break from studying." Well, not really. Nanoha was still going over the material in some part of her head. "Are you here with your..." She thought for a moment, trying to think of what Archer would be to Hayate. "Is she your sister, or...?"

Between 'maid' and 'sister', it was probably less offensive to go with the latter if you weren't sure.