I have DD (just 4), DS1 (2) and DS2 (7.5 months). DD and DS1 play well a lot of the time and fight like cat and dog the remainder of the time. I know this is not unusual. Both have always been brilliant with DS2, loving and sweet. However, I have just overheard DD telling DS2 that she hates him ("I hate you. I hate you. I hate you") very quietly when she had no idea I could hear as I was in a different room. I walked into the room where they were and she had her face right in his as she was saying this. It has totally broken my heart. Please tell me this is not the end of the world