Speak to: Sister, actually. And, well, I did tell you your mother was Arturia Pendragon, so why didn't you work it out before. Plus, there's no reason for her to come after you, and if you see her I'd point her out.
As for the "lie" thing, yes, I guess you're right that it does look that way, but what you said was true, and I thought she might accept you better if you mentioned your mother. Plus, I don't recall you telling her you'd lost your memory....