But the part people do miss is that this is only applied to brothers who "dwell together". So maybe it applies to any male relative, but only male relatives who live in the same household, or at least in the same camp or town?
Could it be that it refers not to the dead, but to the living? In other words, Ruth's husband died in Moab, and nobody sent back home for a kinsman redeemer. But, when Naomi had established herself back in town, that opened up opportunities (or created obligations?), and she sent Ruth to Boaz to invoke the custom.
It's notable in 3:9 that Boaz asks, "Who are you?" We would say he already knew who she was because he had seen and taken note of her. So, his question was more significant, and the obviously abbreviated account of her answer tells us the kind of details she went into. In other words, prior to this talk, it was unknown who Ruth was, so that could be why no action had been taken yet.
But then, he calls it a kindness that she came to him and did not run after younger men, which surely sounds like I need to take back what I said earlier about levirate marriage being automatically obligatory of both parties
@Mark C. Although, he could have said it because no one really knew who Ruth was...she had to make herself known to him before he realized his obligation(?)...and because of that she could have gotten away with not following the custom. Probably also unbeknownst to him, for Ruth, this one was obligatory since Naomi sent her, so he may have been thinking it was her choice to make herself known.
Later, while talking to the nearer kinsman in 4:4, Boaz refers to the buying of the land and acquiring of the attached widow as a "right", not a responsibility. So, perhaps not obligatory for the man either, but again, Boaz is offering himself as a substitute for the other man, so it may be that it would have been obligatory otherwise.
Tough to nail down absolutes from one sketch of one event. If Boaz hadn't taken such a shining to Ruth, it would be easy to say it was obligatory. As it is, there is sufficient doubt.
Side note: it escaped me until now that Jesus's lineage included the hacked marriage of Tamar!