Is it normal that i have a crush on my ex boyfriends brother?

So about 9 months ago I found out my boyfriend of almost a year and a half cheated on me. We broke up but tried to remain friends, it didn't work that well and now we barely talk, except when we have to in class. Anyways I was great friends with him, his brother, and our friend Jacob before we went out. Now it's just his brother, Jacob, and me. My ex isn't really part of our group. Recently I have developed feeling for his brother, we made out the other day. And I don't know if this is completely weird or if it's okay to be with my ex's brother?