Is it normal that i'm interested in my bf's brother.

I've been in this great relationship with my bf for 3 year's now, but... I often catch myself having sexual fantasies about his younger brother, (whos only a year younger then my bf). It started two year's ago when I had this erotic dream about him, when at the time his charm and good looks never crossed my mind. After that dream, everytime I'm around him, I get butterflies. I sometimes fantasize about him while masturbating, and sometimes wish I was riding him instead of my bf. I feel so ashamed, and tried everything to get him out of my head, but he just pops back in everytime I see him or even a photo of him. My bf jokes around saying I'm in love with his brother, because I constantly talk about him(which I never realized). I could never be or do anything with him since I don't want to hurt my bf. Ahh, Does anyone know what to do?