Stop teasing your brother...

I have a question, a survey of sorts, to pose to any readers with early childhood experience, or other insight, into dealing with brotherly teasing. This is an issue I try to curb as much as possible, because little things always end up turning into big things. Then I end up with a headache.

So here is the situation, my four year old, Monkey, has no problem with having girlfriends. For a while, he didn't really have a definition for what a girlfriend is, but I always follow up when he talks about girlfriends by asking, "What is a girlfriend?" His answer to me used to be, "She's a girl and she's my friend." But lately his answer has evolved into, "It has to do with kissing, Mom." I, of course, had to ask, "You kiss, P, Mo, and Ma?" I was relieved when he said, "No, Mom. You can't kiss at school." Also, lately, he's added another girl to his list of "girlfriends".

The new girlfriend is my seven year old's, The Scientist's, friend J. The Scientist has no problem with Monkey saying J is his girlfriend. What The Scientist does have a problem with is Monkey saying that J is The Scientist's girlfriend. J is a girl, she is The Scientist's friend, but at this stage in his psychological development having someone say that someone else is your girlfriend really bothers him. He loves playing with J. We've have a semi-standing playdate every Friday evening swimming and then going out to eat. I need to know how to help him feel comfortable with having J as his friend and still deal with any teasing, which with other little boys seems to be occuring now. With Monkey I'm usually around to make him stop teasing his brother about it and explain the meaning of girlfriend in the context of J as The Scientist's friend. I don't know why Monkey's started this other than it is something he found that he can do to push The Scientist's buttons and sometimes that what he wants to do. Any thoughts/suggestions/comments?