Step siblings

We are probably going to counseling this summer but I am wondering if anyone has any insight. I have an 8 yo d. He has an 8 and a 14 yo s. My d can be annoying and she tries to instigate fights. I know this and we are working on it. 8 yo Ss has such a massive reaction to every little thing she does. He says he can't stand her but he watches her like a hawk and comments on everything she does. Especially if it's not something he would do. We are working on that too. What I don't understand is that he seems to be completely resentful of her very existence sometimes. I have thought and thought and I just can't figure it out. He isn't like this with his other step siblings as his mom's house. What I don't get is when she isn't with us he acts babyish. He rolls around on the floor, sings, talks in weird voices, and does silly things. When she is there, he acts like a cranky old person. He's very serious, quiet, and more mature. Yesterday we went to an event at 14 yo Ss school and he didn't realize we were coming. His response was embarrassing. Both parents got upset with him for yelling "GO AWAY" to d when she went over to say hi to 14 yo. He was told to apologize to her by both his parents and his mom was really mad. I just don't know what to do. I told then they don't have to love each other but they need to act with respect. Any ideas? I just want him to be happy and comfortable in our house. We have them every other weekend and he sees his dad every Tuesday night. We have them about half the summer.

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