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Why is my brother doing this? Not sure what to do?

I'm 23 and my older brother is 28. I had a party at my house about three months ago. A friend of his who is 30 was there. I've known this guy for a long time. At about 3:00 AM, I went up to my room to go to bed and some of the people at the party stayed over night because they were drinking. When I went upstairs a bunch of the guys were playing cards including my brother and his friend. I was laying in bed reading when my brothers friend knocked on the door and came in we sat and talked and one thing lead to another and we ended up having sex and it was wonderful. In the middle of it someone opened the door and saw us going at it. Of course it was embarrassing but we finished.
The next day my brother was giving me attitude. Obviously the person told him and I can get that he was a little ticked. I mean someone tells him he just saw his friend screwing his sister. The thing is this wasn't a one time thing we've been seeing each other ever since and things are going well.
My brother knows we are seeing each other and he's been a jerk to me ever since while he is perfectly fine to my boyfriend/his friend. They hang out no attitude what so ever.
I'm an adult. I was single when I hooked up with his friend. His friend is a widower (his wife passed away two years ago) and I'm the first woman he's been with since. We are happy.
Why is my brother treating me like trash and being okay his friend/my boyfriend?
Why is my brother doing this? Not sure what to do?
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