There are reasonable state interests in not allowing closer than first cousins to be involved in sexual relations, especially if one grew up around the other, related to either undue influence in the relationship (real grooming) or there is a chance of offspring with birth defects (for 1st level relations, it can be close to 40% or more), or both are concerns.
As I said, there is no concern about genetic defects in offspring when the partners are of the same sex, or, for that matter, where at least one of them has chosen to be permanently sterilized.
I am not sure what you mean by "undue influence," which is a term from the law of wills and trusts. It's entirely possible that no family member has done anything whatever to cause either would-be incestuous partner to have a sexual attraction to the other.
I don't really see any reason not to allow case by case bases of marriage between siblings, even blood siblings, if they weren't raised together, since there is little likelihood of undue influence on the relationship.
It's the very nature of laws to apply generally, rather than "case by case." I know so-called liberals like to imagine technocrats who share their antidemocratic views exercising their undoubtedly superior wisdom and morality to determine who gets to do what with his life, or not. But despite Obergefell, we are still a nation of laws, and that can't happen.
There are state interests involved in limiting number of legal spouses as well.
The Chief Justice did not think those state interests were even as strong as those in the case of homosexual marriage. He said that a view of the Constitution that requires this great a leap away from traditional marriage to be recognized as a right must also require that for the even smaller leap of increasing the number of partners beyond two.
They don't involve tradition, "think of the children" with no science to back up any harm to children, or random possibilities of an unknown future, nor should the argument involve "the people voted for this" (since most of these laws were put in place by legislatures).
The majority could not have made more clear than it did in Obergefell that neither tradition nor the will of majorities as expressed through their elected legislators means a damn when five judges know better. The Court can only prevent adult incest and polygamy by fiat, but it just made its willingness to issue arbitrary dictates very clear.
It will revolve around how legal marriage works and protects the spouses from other legal family members, society (in some ways) and each other.
I don't know what that means. If a certain form of non-traditional marriage were already legal, what question of its legality would revolve around how it worked?
The arguments prior to any court challenge should include looking for ways to actually change some marriage laws to accommodate multiple spouses in a marriage
The debate about same-sex marriage that states were engaged in is the very thing the majority cut off in Obergefell. Why should anyone bother with the democratic process when it comes to other forms of non-traditional marriage, if the Supreme Court is only going to substitute the personal views of a handful of judges for the judgments of majorities anyway?
As with any SC battle, such cases would be decided on their own arguments, both for and against, not mainly on previous cases.
Again, I don't know what that means. The Supreme Court regularly considers its previous decisions in deciding the case before it. In Casey in 1992, Anthony Kennedy sure as hell made a big deal out of stare decisis when he needed a convenient excuse not to overrule Roe. Or maybe you are trying to say that neither legal reasoning nor the Constitution would have anything more to do with the Court's decision in a future case involving a challenge to state laws against adult incest or polygamy, than it did in Obergefell.
The notion that Obergefell was decided on the arguments is laughable. You might want to read more about how substantive due process works. Forget about the subject matter--just as a Supreme Court decision, the quality of this monstrosity is right down there with Roe v. Wade, another notorious substantive due process piece of junk.