When my kids were little and tried to tattle on each other, I would try to use it as an opportunity to reinforce helping each other in a spirit of love. So when the tattler told me what was going on, I would ask the tattler, “And did you remind her in a kind voice what she should do instead?” If yes, I would thank the child and go deal with it. If no, I would encourage that child to go help the other one in that way. And usually I would accompany her to supervise the conversation and encourage peacemaking.
Usually I could tell the spirit of the tattler by the tone of voice she used. My goal was to cultivate a spirit of genuine concern for the sibling’s well being.