I can't imagine Rachnera being the serving type. It could be a roleplay kind of thing with her though. During the day she's the attentive maid, while at night she owns you. Maybe she likes the outfit, as they do both look very good in it.

Cerea would probably just like another way to serve her master. Plus, it'd be a great chance to bring herself closer. After all, how many tales have the lonely master fall for his lovely maid?