Thanks so much for help! I tried to keep it short as advised.
((most recent version))
Sixteen-year-old Sam Croft is dead, but he can’t remember
everdying. Good hook.
His sudden arrival in Heaven with his parents makes him suspicious (I would add a little bit more context by telling me who who finds it suspicious and why), but he can’t get to the truth as all negative memories are erased in Heaven.
However, as time passes, Sam starts remembering his sister, Cara—who, oddly, no one else can (I feel like you need to add a word here, because this sentence feels like you cut it off a word too short.). His search for her leads him straight to Hell (I am finding it hard to visualize this search because I don't know anything about what heaven & hell looks like in your world.), where all his memories return, including that of his death.
Cara had them all murdered. Good.
Hurt and betrayed, Sam can’t deny Cara had grown cold back on Earth (This doesn't sound right to me although I understand the message you are trying to convey. I would say "became colder and __distant/or maybe another word that will help describe her__ back on earth.), but how could it have escalated to murder? (I would reword this somehow into a statement.) Whatever her motivations, she isn't done, as Sam discovers she's on her way to invade Heaven. Her target?
As a pacifist,Sam tries stopping her (Pacifist or not, of course Sam would like to stop her. Any normal human being would.), but words can't hold her, and the sight of him only angers her. Cara won't give up without a terriblefight. A fight Sam can only win by being as ruthless as she is. Soon, violence and hatred begin corrupting him, and his need to protect his family turns into a desire to hurt Cara and avenge their deaths.
Sam can’t succumb to the darkness within him, or he’ll be locked out of Heaven forever. Very good last sentence, but I am wondering how Cara can invade heaven if she is a murderer. Right now, this looks like a plot-hole in your book, and this might be enough reason for an agent to reject this query.
This is a lot better than the last version I reviewed, but more work still needs to be done. In addition to the comments above, I think you need to reveal or at least hint at Cara's motivations or the reason why she has become more cold. It is important for the agent to see this, to make sure your character has emotional truth. I.e. there is a good reason for this transformation.