It's better to have allies than enemies, for Ati's sake. If the maid had kept viewing Selene as an adversary, she might have unwittingly hurt Ati in the process of making things difficult for Selene. But if they both work for the same cause, they can help each other to make Ati happier.
That being said, if Ati does grow into a decent person, she would like Selene to be happy as well. Although it might take great many years for her to be mature enough to possess such thoughts. But then again, this is fiction, so she might push for Selene's happiness much sooner than a real child.