C. F. Keil and Franz Delitzsch on Leviticus 18:14

Lev 18:14

So, again, with the wife of the father’s brother, because the nakedness of the uncle was thereby uncovered. The threat held out in Lev 20:19 and Lev 20:20 against the alliances prohibited in Lev 18:12-14, is that the persons concerned should bear their iniquity or sin, i.e., should suffer punishment in consequence (see at Lev 5:1); and in the last case it is stated that they should die childless. From this it is obvious that sexual connection with the sister of either father or mother was not to be punished with death by the magistrate, but would be punished with disease by God Himself.