I'm surprised that some are calling the "older sibling dislikes baby" element a trope, or overused. There are some things that show up in stories because they're common in real life, and that's one of those things. I think most kids come around to the new sib sooner or later, but especially in that first year while the infant is breaking the parent's sleep up and making them be zombies, and there are round the clock feedings, and everyone's giving all their attention to the newcomer, there is lots of potential for jealousy. A parent who is on top of their game will probably think ahead about this potential and try to offset it as much as possible by finding ways to make the older kids feel special--in those cases maybe there are less hard feelings, through added effort on the parents.
I saw the body-swap thing pretty early on--I didn't think it was too obvious necessarily, but there were ample clues. I had an inkling of how it might end, I'd gotten an impression from the brother's reaction to the baby that violence might result. The most tragic thing about the whole thing is that, even though the brother with the information from this narrative is probably killing a parasitic monster, he will be seen as killing an ordinary baby. A developmentally-delayed child is tolerated, a developmentally-delayed child that has killed a baby is going to have a very hard life.
This story was very well done, I thought.