So, if the story is rife with ghosts, AND the brothers of the heroine die, the question of whether they will be ghosts will arise.
Whether they are or not. Whether the heroine ever learns why, or not.
I'm leaning toward -- not. Too much complication on a tangent. Also, come to think of it, there has to be some rule because while the characters will meet a lot of ghosts, that's only because they have been accumulating for years. They live in a city. The number of accumulated ghosts would rapidly grow very large if they were all ghosts.
It may just be a matter of who kills you. Since I think all of them were murdered, but even then the number is too large unless only some murderers count. Then I have to work out who. Most of the murderers will not feature in the story, because there are most ghosts than that. Something necromantic involved.