I fail to understand what is your problem on this. Your brother and the maid both are adult and they are having consenting penetrative or non penetrative sex. What is bothering you? Let them do what they are doing, just stay busy with your job. You are adult, if you have sex with someone, can your brother say anything? Nothing at all.
Whether she blakmails of not, thats your brothers risk, he is taking that. If he really is in love with this maid, you may end up accepting a marrage and calling her sister in law. Is this caste or social status bothering you?
In number of hospitals doctors get married to murse, Boss marries to receptionist, I dont see what is bothering you here. Two consenting adults mentally and physically attracted, let them stay in peace. Dont poke your nose on other people's privacy, else soon someone will also poke their nose on your private life.