I always turn to wikipedia when I want spoilers for films I don't want to see, but have enough morbid interest to want to know the ending.
Orphan was one such movie and I read through the wiki info, which is a good full summary of the film, to figure out who lives, who dies, the ending, all that.
So...
[spoiler:2aqhv48u]Yeah. She's a 33-year old woman with a disease that doesn't make her age. Basically we're talking Baby-Doll from Batman (maybe just the cartoon series. I don't know if she was in the comics). She's also psychotic and has killed dozens of people before the start of the film and just hides in plain sight by pretending to be an orphan and laying low between killings.
In the film she breaks a young girl's leg by pushing her off a slide, caves a nun's head in with a hammer, and tries to seduce her new "daddy" John and eventually just stabs him to death when he freaks out. Then she goes after the mute/deaf kid in the family with a handgun. The mother arrives to the rescue and they have a fight out on some thin ice. The ice breaks and both she and Esther go under. The mother crawls out, but Esther pulls herself partially out as well and tries to use the little girl persona again by crying out "don't let me drown mommy". That's when the mother says "I'm not your fucking mommy" and gives her a swift kick in the head to send her back to a watery grave.[/spoiler:2aqhv48u]