WHAAAA

Yeah like bruh, move you frickin maid, like why would she bring her in MC's room, wasn't there any reception room when guest arrives? Suspicious ass hell

That maid is Maria, who's works as a spy. True the sister is obviously sus but it's a smart maneuver for a) gaining more info about what the sister's plans and b) baiting the sister into committing a crime.
And it's not like they left Eva alone with the sister since it was Rudolph who went to get Alan, so Maria must have been on standby in case things went pear shaped.