Yeah, the story IS quite complicated, and I might have spoiled the entire mystery by telling you that the person everyone think is murdered actually is alive. But it's still a good read.
The first of the two MC is a young, rough rebel sort of girl. She is really good with computers and very smart.
The other MC is a journalist who everyone starts to hate because he posted some really controversial articles. When he is contacted by an old millionaire to solve a fifty year old murder case, he says yes to get away from it all. The other MC helps him out and the mystery slowly unfolds.
But it's a super gritty, violent book. There will be passages with rape and torture, so don't read it if you don't want to read stuff like that.
The mystery part of it is REALLY good though. Stieg Larsson is a great writer in my opinion. Such a shame he died.
If you're going to pull the same ending though, that the sister still IS alive, make sure you have a good reason why she never contacted her family and why she went away in the first place, because she must have had a real good reason to make her entire family think she was dead if she wasn't. Maybe she was kidnapped and COULDN'T contact anyone. Or she someone, like the mafia, threatened to kill her and she couldn't even let her family know she was still alive, because then the mafia would have found out.