What should i do or say about my ex husbands step daughter?

My ex husband has married a woman with three kids. The youngest is in the same grade as one of our daughters. They see each other on the playground at recess. The step daughter is not very well behaved and he has told me his self that she is the spawn of the devil. My problem is that the step daughter is telling my daughter stuff about her dad that isnt true. This last time is the worst, she told my daughter that her daddy got shot in the head and died. My little girl was absolutely distraught. The school counseler had to call me to get her settled down because she was so understandably upset. I have talked to him about this several times in a mature nice tone. his answer is they are just kids, but our kids are the same age and my daughter knows better and has been taught to do better and would never do something like this. I do not want to be mean and cause trouble but it really upsets me when my kids are upset. They come home from his house and start crying because they were picked on all weekend. I just do not know wat else I could say to him or do that would not cause a huge fight between me him and his wife. I know kids will be kids but I believe this has gone a bit too far and has become too repititious. I know that the step daughter is on medication because my girls have told me that she has to take a "be good pill". Apparently its not working or they do not give it too her every day.