Hysterical, slippery slope logical fallacy inane equine excrement. It has been about six years since the Obergfell ruling. How many people have tried to marry a sibling, or a child, or their dog citing that ruling?
Gay people made their case for marriage by challenging the states to come up with a compelling government reason, or at least a rational basis to deny unrelated, consenting adults of the same sex marriage. The states failed miserably.
If someone wished to further expand the definition of marriage to include close relatives or animals they are welcome to pursue it through the judicial and/or legislative process. However, the issues would be different and for the states to show that there are compelling reasons to not allow such unions would likely be far easier.
Lastly, the Obergfell decision SPECIFICALLY states that the right for same sex couple to marry will be in accordance with current state laws governing marriage between opposite sex coupls. Thoes laws prohibit siblings and parrent -child marriage. So get a ******* grip!