leecappella said:
No one seems to be touching on the issue that God accepted such a thing. Does this sound like God or humankind adding God's name to the custom?
Interesting question. As I read the OT, this seems to have been a custom that became incorporated into the Mosaic Law. I think there are a number of customs that were "understood" as the social background of the Mosaic Law. My personal view is that these were neither endorsed by God for all times and places, nor condemned by God. However, a number of them were moderated by God, and we should pay attention to these, as giving some insight into God's view of these social customs and institutions.
For example, the Mosaic Law assumes the existence of slavery, but it limits the duration of slavery for a male Israelite slave and limits the sexual rights of a master WRT female slaves, Israelite or alien. If a master or his son has sex with a female slave, she thereby obtains all the rights of a wife, and can no longer be sold.
Based on the Tamar story, it is apparent that the levirate custom prexisted the Moasaic Law, but as someone else pointed out, it was incorporated into it. We see it mentioned again in Ruth, and can gather there that if the deceased husband had no brother and the widow wished to remarry, the closest living male relative of the husband basically "had first dibs," but could release her through a legal ritual.
I think your opinion is that levirate marriage seems to you too close to incest for you to feel comfortable with the idea, and you think of it somewhat like slavery - an evil custom that God allowed to continue for a while.
I don't look at it that way, but as a social welfare system, designed as a way to see that childless widows still belonged to the family and had someone to care for them. Other societies have different mechanisms for providing for the elderly and vulnerable. This one is probably no better and no worse than other means that might be devised. It assures that there will be (if possible) a subsequent generation to care for the widow in her old age.