Whilst not assuming that every sibling couple would seek to have children, there's pretty substantial scientific evidence that inbreeding is not great for the children. Procreative relationships between close relatives could be argued to be unethical because of the likelihood of genetic problems. They may well hurt the community. It is not as simple as 'people get to do whatever they like because they're individuals'. Individual choices impact the community, and the well being of the whole must also be considered. Inbreeding is directly harmful to communities.
On saying that, non-procreative or non-heterosexual sibling relationships? It doesn't hurt anyone, from an objective ethical standpoint. If they're consenting, who has the right to impinge? Encouraging adoption amongst sibling couples is a perfectly feasible strategy, if they want kids.
In regards to the original topic: Gay marriage has no logical relationship to incest, or child abuse, or even heterosexual relationships. I'm not sure why the comparison has even been suggested. It doesn't harm anyone. Offence and belief... are not the same as harm. Change of a social paradigm is not the same as harm. Offense, faith and reluctance to change are not valid reasons to deprive 5 - 10% of the human population essential human rights, privileges, and responsibilities.
No religious zealot was harmed in the making of this marriage.