an arranged marriage between two rich families however Yuuri's parents died before they could introduce him to Lilith but Lilith still wants to go through with it anyway even if Yuuri isn't rich anymore (although I suspect there may be a trust fund). Lilith decided that the best way for Yuuri to get used to her was by acting as a maid instead of his future wife.
when you consider the age gap between them theory seems less plausible. yuri's parents surely won't give their son to someone way older than him. i think maybe she feels responsible in some way for his parent's death.