Me? I think marriage should be taken out of the government’s hands entirely. The government ought to deal in civil unions only, and leave marriage (and the definition thereof) to religions.
The case against brother/sister unions is strongly rooted in the danger to the health of the offspring of such pairings. I’m not sure about brother/brother. i’m certainly no fan of the idea, but if a church wanted to perform a marriage ceremony for two brothers, I can’t imagine it affecting me (and children wouldn’t be possible, so I’m not sure who would be affected).