First of all, let me just say that I could never do that to anybody, and I don't understand how other people could do it so cavalierly. That being said, you need to find out if this was just a one time thing with the older brother, or if he is even willing to have a relationship with you. You called him your new love, so there had to have been other encounters before this with him. Since you cheated on your bf with his brother, his brother knows your a cheater; like they say, once a cheat, always a cheat. I sense that something else must have happened because you wouldn't just screw up a great relationship overall for something like sex with a bigger penis; that sounds absurd. Perhaps you were just looking for an escape and his brother just happened to be there. Regardless of if you start a relationship with the older brother or not, you need to tell your (soon to be ex) boyfriend. The longer you hide it, the harder it's going to be to fess up to, and the more it will hurt.