I don't think just one thing was going on, here. Overall, though, I'm with @Mira on this, and @JTass.
I expect Mab had given orders to keep Harry ignorant for a bit longer, and Maeve "had to" obey (Nemfection would let her disobey, but she's still pretending to be "undercover" and hiding her freedom from Winter edicts).
So, Maeve was "obediently" feigning that she had little or no knowledge of Sarisssa; but also leaning-in on the "crazed nympho slut" image she likes to portray (insinuating all sorts of erotic delights on offer); plus (from Sarissa's obvious reaction) making it into a threat; and (as another layer hidden from Harry) playing an incestuous head-game with Sarissa.
It's all kind of... petty and trivial. But it's part and parcel with Maeve's self-image as a major player in Winter's layers-upon-layers of misdirection / multiply-redundant planning & goals, whereas she's actually kind of bad at all that (Winter Lady's Mantle gives her a lot of power & faerie glamour, but the mind directing all that power is... well... petty and trivial).