Well, it's not really a confession... it's... well, it's in a mystery novel. Basically, a girl (Mattie) finds out that basically everything she knows about her own past are lies. She gets her adoptive parents (Cynthia and Sir) to explain everything. Mattie is sitting in her adoptive sister's (Elara's) room, when Cynthia and Sir come in. I'm not sure how I should write this explanation without it coming to the cliche "Mattie, I have something very important to tell you" "I know this will come as a big shock" kind of stuff. How do I make it original and believable?
There are as many ways as there are writers. I wouldn't give out everything at once, or in a straight line. Whether through evasiveness, or reluctance to hurt or overburden the girl, I would go for plenty of breaks, changes of subject, and incomplete explanations. Consider making the full story require several such conversations instead of only one. But that is my approach. Yours should have your own style to it.
I'd be careful with the subject of adoption. These days, most children who were adopted know that fact pretty early on -- some parents tell them from the time they are babies. Obviously they don't understand when they are babies, but the point is they are told throughout their lives so it isn't a shock. This is different from what used to happen in the 40s, 50s, 60s and somewhat in the 70s and 80s. If your story is set in the past that would be fine, since the conventional wisdom was for everyone involved in adoption to pretend it didn't exist. But if the story is told in the present, there would have to be some reason why the fact was hidden and it would not make the parents look very good. The reason why the adoption was kept a secret would have to be fully explained. Also, it is unlikely the parents would suddenly feel that the girl had to find out right this minute that she was adopted and storm into the sister's room. If there's some reason why the disclosure of information needed to happen suddenly and unexpectedly, make the explanation flow from the occurrence of that event that necessitates telling Mattie the information immediately.
the best way to approach this confession is to plan it so it's the most entertaining and believable. this is something only you can know. for example you might want to go with everyone in a room as the parents tell the truth if you character is the kind of person to flip out a start screaming whip out a knife and stab someone. lol i know this is terrible just an example. but on the other hand you might consider give the confession as a letter, if this character is more of the quiet type. the character would read if you want a more personal, and emotional feel to the scene. this is for you to decide, you know your characters and how they'll react to this, you know what you want to make the reader feel. these are two stupid ideas only to express concepts, i'm sure you can come up with something good on your own that will exactly reflect the emotion you are trying to convey
Sorry if I made this unclear... Mattie knows that she is adopted (it happened very recently, actually). It is something else that they are explaining. But thanks for the advice!