In these two legal cases ([#1] a nephew had sexual relations with his by-marriage aunt [vs. 20]; and [#2] a brother had sexual relations with his sister-in-law [vs. 21]), we – first of all - have not to conclude that is envisaged a death-sentence for the new couple, because the Bible global context and the logic (not the plain text, regrettably) urge us to conclude that the ‘woman’ at issue was a widow, really.
Besides, if the uncle/brother were living, these two cases would be rightly situated within the bounds of adultery > death-sentence, it is not?
Moreover, the case #2 was desirable – from God’s viewpoint - in a given situation (this case was called יבם [ibm], see e. g. Gen 38:8).
Finally, the verse 21 does not speak about a ‘death’ at all, but says: “They will become without sons (or, ‘descendants’) [oririm ieiu]".
So, where we may spot the infamy/filthy thing/impurity (נדה [nde], vs. 21) in these two legal cases?
The only discriminant I see – I might be wrong, obviously – is the presence, or the absence of any sons of the ‘original couple’.
Then, if the ‘original couple’ had not any sons, in both cases (#1, or #2) the new marriage union was perfectly legal. They suffer no consequences. In other words, the verses 20 and 21, did not speak about these legal unions.
If – instead – the ‘original couple’ had any sons, these cases (#1, or #2) represent a ‘filthy thing’ (נדה). So, the verses 20 and 21 do speak about these illegal unions, only.
However, it seems to me, that also in these cases the ‘new’ couple were not sentenced to death (as I say before, this is demonstrated also by the fact that the verse 21 does not mention ‘death’, at all). The sole consequence is penned in the TANAKH with the term ערירים (oririm), that means ‘without sons’, or, ‘heirless’.
But, what involves this term?
Avoiding the cruel and bizarre explanation of some commentator (the sons of the ‘new’ couple must be killed along with their parents...), the only conclusion in harmony with the God’s justice and love (as it is described in the Bible) is that – in these cases – every sons of the ‘new’ couple (where the woman had yet son/sons from the previous marriage, as I say before) were not considered their legal descendants, but they did legally belong to the dead uncle/brother, with all the inheritance-related consequences.
I hope these information will be useful for your research.