People trouble people, and the scars linger for years, decades. Sometimes we simply don't know when to stop our troubling of one another. It begins as teasing, maybe. For years, sisters or brothers can be in the same room, for family celebrations or family grieving, and not speak to one another.
Life is sometimes just plain broken.
Let me hasten to add that it happens to everyone, Christians not being exempt. We are not a crowd that through gift and grace and force of will rise above the ways of woe in the world. To be Christian does not mean that day by day in every way we are getting better and better. Look around you. Better yet, look in the mirror.