Here you have two elements, the false death, but also The double. Therefore, as I see it, the only way for it to be an endearing story, that moves the reader, is that effectively the double is even better than the subject it is imitating and is transformed at the same time into a kind of A Christmas Carol, in which the character being replaced sees the good in the villain and learns a lesson, so that being saved gives him a second chance to be a better person.
I see the problem in the argument. For one thing, how is it possible that there is a shapeshifter? However, we are going to do without the explanation, because we still have resources with which to build a good story, so that if you give them an adequate treatment, indeed, the reader will no longer care to know the explanation of the shapeshifter.
I would say that to we, the writers, the readers grant us some wild cards; But we can't go overboard, either. Even so, the initial situation does not give us the possibility of the wild card. Here the reader will not forgive us for any mistakes. Why does the shapeshifter kill the protagonist's brother? Nobody kills another person just because. Hundreds of murderers who are loose on the street take the subway every day, but that is not why they are pushing old women onto the tracks. They are even nice, polite, give generous tips, people have no idea that they are criminals. In fact, and this happens in real life, the only type of criminal that the police are truly afraid of, because they know that they will never be able to catch him, is the spontaneous criminal. But this does not happen in your story. Therefore, the crime motive must respond to a logic.
For the rest, the items according to their true importance. The victim is almost always a hindrance and by eliminating it he allows access to the object that the villain wants to get. But in your story the villain takes the place of the victim, and that rules out the probable strategic, business, political or military importance of him.
Here we are talking about position.
In other words, what is really important about taking the place of the protagonist's brother? For example, if I were the shapeshifter myself, why am I dying to be that boy's brother?
Or the sister ... Woman.
That is the problem that I see with your argument. It is a matter of seeing reality. For example, cross the street and go to the bakery on the corner. Don't worry, you don't need an umbrella, it's stopped raining. But I can bet you won't find brother cakes. "What is that?," the manager will tell you. Most extravagant thing she has never heard of her.
But you will find wedding cakes.
The way I see it, it is the only thing that justifies the fact that the villain is even willing to die in the event that his plans fail. He wants to take the place of the victim because he is not the brother of the protagonist. He is her husband. Which, incidentally, in order not to be too modern, advises that the protagonist be a girl. But following the example of the wedding cake, one of the two must be of the opposite sex.
Furthermore, the definition of the MC is wrong. Because whom is going to have the whole movie rain, the one who from the beginning the family employees are going to look at him between eyebrows, as if they find that something is wrong (the tradition of nosy owls of domestics is biblical), is the villain. That is the real MC of your story.
Anyway, those are my humble advice.
