Seriously? This almost happened to me just before Christmas! I did the honorable thing and stuck with the literal "Bros before hoes" mentality. I told her I've already got another girl and so the urge passed. You really need to stick by your brother, even if you hate his guts. Him finding out about you boinking an ex could drive a wedge between the both of you that could last for years, if not forever. The only loophole to this is if he actually says to you: "Yeah you can fuck my ex." Otherwise, you should just leave it alone.