I see you have already reached a conclusion on this issue. Personally, though, I would not attend the 'wedding'. My attendance would imply that I am there supporting or giving my blessing for an 'union' that is an abomination to God. As a Christian, I try to do the Lord's will, regardless if others agree with it. Basically, on one hand I would be saying I am opposed to homosexual 'marriage' and their agenda, but on the other hand I would be celebrating a homosexual 'marriage' with my cousin.
The responses on this thread have surprised me because blood relations have come up as a reason to support immorality.