Uh....are you seducing your maid, Annie? Again?
Well, this does work though. Pick the most inexperienced and immature maid so she's easier to control and maybe easier to win over.
Aria is...not a Mary Sue. She's clever, resourceful, spiteful, and desperate. All traits that make for an excellent villainess.
The term Mary Sue isn't even used accurately anyway. A Mary Sue is a character that twists and distorts the story until the plot serves her and things don't make sense. Contrivances, illogical deductions that are somehow correct, and characters that don't react like people with this person in particular are what make them a Sue.