In this isekai a god pulls a girl to take over the role of a villainess in this new world where she has to "act" the role and fulfill her purpose in order to save her sister's life in her own world. She accomplishes this but instead of being banished like she hoped she is turned into a maid.
Why does being a villainess in that world save her sister? -She essentially makes a deal with the god of the fantasy world, her sister in modern Japan (i presume) is sick and dying, the deal entails for her to fulfill the role of villainess in the fantasy world as to "not disturb major events in the timeline of this world or something like that" is kinda vague, mind you all of this happens within the first few chapters of the manga, also she gets her engagement annulled because she was bullying the heroine, however since she never did anything violent or harmful, she is stripped of her noble title and made a maid for the palace, (she wanted to be banished).
Another detail that i remember is that when she was demoted to maid there was a head maid that whipped her arms as a form of punishment and a knight (captain?) gave her some medicine. that is all i can remember for now.