What if I said that I had a dream that I made out with my brothers girlfriend?
Did the dream seem realistic?
Yes, it woke me up in the middle of the night and I was all excited and I am ready to get with her.
If your brother's girlfriend is around you a lot, it is normal to dream about her. You shouldn't worry or put too much stock into dreams, they don't have anything to do with our real lives, but it wouldn't be correct for you to pursue anything with her because it would devastate your brother.
I still want to try it with her.
That would not be a smart idea to try and get with your brother's gf. Think of how your brother would feel if you went behind his back. It is not worth losing your family over this.