who is the maid advisor to mielle?
She basically loves Mielle like a daughter and can't accept that Aria is a prostitute's daughter so she's the mastermind behind most of the major schemes. Gets caught and meets death later on.
Was mielle really nasty or was she goaded into it?
Yeah she basically couldn't tolerate Aria being of low origins, and prettier than her etc, some reasons but it was exacerbated by the maid. In the end though, she loses her sense of reason and pretty much does unforgivable stuff.
What happened at the end? To her mother, father, sister, brother and Oscar?
Mother meets good end basically lives happily ever after with money, if you remember reading in the manga early on she talks about one man who promised to marry her and never came back, that was basically Aria's father so he comes back and they have happily ever after, the father, sister and brother all meet bad endings. Oscar falls for her pretty hard but since she has no feelings, he end up with a pitiful ending as well.
The hourglass is kind of like a cheat special power that occurs when she is on the verge of death. She possesses this power, because her father turns out to be a real powerful person at the end, and she ultimately has a happily ever after with the prince.