Marriage isn't about making babies, so I'm going to avoid the arguments that involve genetic defects. They're perfectly good reasons not to have children with your siblings, but using that argument presumes that the point of marriage is to make babies. I doubt sterile couples consider their marriages pointless.
What rights, if any, do married couples posses that siblings lack?
While they do have a number, like hospital visitation, they do not have the right in most states (last I checked, being these rules are not well known) to even become intimate with each other without also becoming sex offenders, even though both siblings are over 18. They also lack social status, as in saying that if they made their union known they would be targeted by wide spread discrimination. They also are not able to adopt children as a family, it would be one or the others, but not both's.
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