I loved this story, though I had to forcibly dismiss the skeptic in me who wanted to say the protagonist had forgotten her brother out of grief and the visit to the bell started the healing process. Begone ye foul skeptic! Skeptics have no place evaluating fantasy fiction! (Also, skeptics are boring and their mothers dress them funny.)
My preferred interpretation settled on this: The war bells were a most unique and peculiar necromantic tool: One that did not merely call back the spirits of the dead, but summoned them into existence in the first place, knitting them into this world with all the myriad of consequences this implies.
What consequences you ask? Well consider the white cat. Did we hear one word about this cat before the protagonist remembered her brother saving it? We did not. One might easily presume that there was no cat until the ghost brother was made real by remembrance. Now suddenly the family has had a cat for years. But every human lifetime has a plethora of consequences like this one. How very many things may have changed far into the past because a ghost who did not previously exist has now been made real along with his past.
But what a terrible and unpredictable weapon of war these bells would be. No doubt the bell keepers wished to conjure the memories of soldiers on their own side. Soldiers who died heroically serving their own cause. But once unleashed, these forces echo across the countryside, raising the spirits not just on one side, but both, altering the past again and again in complex ways that perhaps no mere mortals could contain or direct. We never do find out what happened to the bell's fanatical keepers. Could their own magic have been their undoing? Did they fall victim to now fallen heroes who died striking down the enemy bell keepers? It may be impossible to ever know for sure.
I do know however that I loved this particular plot device that made a beautiful and memorably original story.