Here is an example of his behavior:
Today I picked them both up from school. My daughter gets into the car crying, with my son right behind her. My son had pushed her because she had embarrased him. I asked what she had done to embarrass him. He told me that she tried to"cuddle him" in front of his friends, and that had embarrassed him enough for him to become angry and push her. Now, I'm sure my daughter was just happy to see him, and wanted to give him a hug, (she loves hugs) but being rejected she could only cry.
I tried to explain to my son that his sister loves him, and was happy to see him, but he could care less. He was more worried about what his friends thought, than how is sister was feeling. How do we as parents get around this peer pressure?
I wouldn't make him the bad guy here, although I would clearly and firmly indicate what behavior directed towards his sister is unacceptable. I would have two sets of conversations, two separate one-on- ones with each of them and a follow up to those with both of them together.
The private individual conversations should focus on empathizing with each one's point of view and needs; she with her needs to connect closely with her brother and he with his needs to distance himself from his sister's needs for him (and any meaningful contact with any girls for that matter) and desire to show her affection to him. Talk to each one with respect for their feelings and an understanding of how it's sometimes difficult to have a younger sister/older brother. There are some good examples of how to conduct these conversations in" Siblings Without Rivalry" by Faber and Mazlish.
Your talk with both of them together should be a summing up of each one's needs and feelings and your suggestions for how both their needs can be honored (that's not always entirely possible). Expectations about the common courtesy you expect they'll show each other should be stated. Your daughter should in no way be discouraged from being her natural, loving self but should be made aware that at this particular time her brother doesn't like doing anything with any girls, not just her. You don't get around peer pressure of this sort at this age and stage (there's plenty more to follow). You do show how you treat and expect him to treat everyone by your own example and words. He will move through all these powerful stages of peer influence (as will your daughter) with you two parents providing him the living, breathing examples of the values you cherish.
I know your heart breaks for her and I know you want to talk some kindness and sensitivity into him but I'm sure you'll handle this well, for the good of all.