Maybe we would say, "whether you are required to marry your brother's widow" - it would be a somewhat unusual choice in modern western cultures, but not forbidden.
I didn't think we were restricting the discussion to modern western cultures. In some cultures, marrying your brother's widow has been required; we generally see this as being part of the social safety net in that culture.
In some cultures, marrying your brother's widow is prohibited because it counts as incest.
I don't know how I would describe the state of this variable in modern western cultures. In modern America, I tend to think it would raise eyebrows and people would be uncomfortable with you.
So looking at the wiki entry on Levirate marriage[1] I see that it's been more common than I remembered, a quick look around didn't turn up any cultures where it would be flatly forbidden as incestuous (I picked modern western as an example where like you say it would seem unusual, but I can also imagine scenarios where it'd be seen as heartwarming, making sure the widow and children were housed and fed etc.)
> Despite the great store Deuteronomy places on this practice, the Priestly Torah seems not to endorse yibbum. It legislates the following blanket prohibition:
> [Lev 18:16] Do not uncover the nakedness of your brother’s wife; it is the nakedness of your brother.
> Here the text forbids a brother from marrying his brother’s wife, ostensibly even after the brother has died. No exception is noted for the brother’s childless widow.
> This problem so bothered the Sages, that they list it within a collection of contradictions in the Torah whose reconciliation is a feat so intangible “that the mouth is unable to utter and the ear unable to hear"
> The cumulative evidence suggests that P/H did not consider yibbum an option, and likely repudiated it. Perhaps the Priestly authors did not feel that incest should have any exceptions—yibbum, which involved a woman marrying her brother-in-law, is at its core incest with an “indulgence.”[5]
> [5] The Sages seem aware of this problem. See, for instance, Tosefta Yebamot 6:10 (cf. Yeb. 39b):
> A man who goes in unto his childless sister-in-law for the sake of beauty or for the sake of property is to be considered as if he is committing incest
That's certainly interesting, both required and forbidden is a case I hadn't expected. I suppose it'd be more practical then to look at historical records and see if the practice was non-existent, or just rare (presumably it wouldn't ever have been common).
