>Secondly, is marriage really a civil right?

Yes.

>If you say yes, explain why campaigners aren't fighting to allow brother to marry sister, mother to marry son, or polygamous marriages which have a long history in many cultures?

I think those rights should exist and I'd be shocked if there aren't organizations fighting for the legalization of polygamy. The incestuous marriages you mentioned are probably not represented because those practices are on the one hand, taboo, and on the other hand, practiced by such a small minority that they're unable to materialize a voice.