keep in mind, that Onan, levirate marriages weren't about love or anything like that.

Like most (all?) cultures at the time, women were property, and couldn't own property.

Technically, Er's firstborn son would have inherited everything and then "taken care of" everyone else. Or rather, Tamar, would, on the son's behalf. but Er got smote by god for being a particular kind of dick before he had a first born son.

So in typical religious fruitcake fashion, instead of fixing the original problem- that Tamar had no way of inheriting anything or supporting her self- they decided the obvious solution was for the brother-in-law to get the widow pregnant.

of course Tamar was the same age as the third brother; who needed to grow up a bit before she could marry him... (ew, right?) so instead she gets sent back to her own family and- eventually- becomes a camp follower of Judah's.... and, ah. yeah.