Being able to marry another consenting adult of their choice would about cover it.
Even if that consenting adults happens to be a sibling?
Why not, what's the harm to you or society? The only possible concern is the higher risk of genetic deformities any children biological siblings might produce, but that's ******* each other not getting married. And that is much closer to a rational basis than denying the rights of two gay people who without the help of the opposite sex aren't going to produce any children at all.
So the reasons you have make it OK to limit something you call a right? Since you admit rights aren't absolute, you should have no problem with marriage being defined as between a man and a woman. If you do, you're saying that marriage is a right when you think it should be and can be limited when you don't. Hypocrite.
This has been discussed fairly often in the courts.
Marriage is a right- and as such- can only be denied by a compelling state interest, and only when that denying that right accomplishes that state interest.
For example- Wisconsin had a law forbidding men who owed child support from getting married.
The court pointed out that while the State had an interest in ensuring that parents pay for their children, denying those men their right to marriage did not accomplish the State's interest.
Similarly another state did not allow for inmates to marry- but could provide no compelling reason why inmates should not be allowed their right to marriage- so that law was overturned also.
Such is the issue with same gender marriage- if the state wants to deny same gender couples the right to marry, it must provide a compelling state interest to treat them differently- and the states have not been able to provide a compelling interest so far.