What I still don't understand

So at the end of the movie we understand that the ghost of the mother is the one doing all the killings, to protect and 'preserve' the family. Well....WHY? The mother is the one who got abused and locked up in the ceiling, she then got revenge against all the family who wronged her, so why in hell does she keep killing people as a ghost and try to lure her boy into the same pattern?

And it's really unclear why she decided to kill her twin sister all of a sudden, usually I can manage to make sense in those perpetual unclear endings in asian horror movies but this one is either a huge plot hole or I suddenly am pretty dumb...

IMO the movie should end by the spirit of the mother getting revenge on the family and then lure her boy far away from the house to protect him from the curse, not the other way around. It's kind of ridiculous, it's like 'oh you locked me up and tortured me, giving my blood to baby fetuses, I'm gonna kill you all!...so I can do the same thing with my kid' loll