I am confused by your question. Are you asking whether it's a sin to have sex with your brother after he's married? Or whether it's a sin to have sex with your brother? As I'm not religious, I don't..think of things in terms of whether they are sinful or not. However, such incestuous relationships are often..abusive and therefore psychologically damaging. I suggest you speak to a counselor of some sort who can help you untangle what this sexual relationship with your brother is all about. It might give you the helpful clarity you need to move on...

Forget about being a sin or not. It is illegal in a big way. You could end up in jail and also registered as a sex offender. If word gets out (and it will sooner or later; trust me) you will be in deep doodoo; it only takes a social worker who wants to keep her license clean, to take the initiative to report you. I would find a good criminal lawyer and consult with him. God is busy in Haiti dont worry about what he thinks...

first off...i cant believe you had sex with your brother..that is gross... you should stop. ..
and second..NO of course you caNNOT continue to have sex with him after he gets married. my dear you need a lot of help. stay away from this brother of yours...
im guessing your pretty young too... try your best to go to college, and stay away from guys all together until you are finished :) you will thank yourself in the future...