John Gill Commentary Deuteronomy 22:30

"A man shall not take his father`s wife, and shall not uncover his father`s skirt." — Deuteronomy 22:30 (ASV)

A man shall not take his father's wife
Not marry her, whether his own mother, or a stepmother; or even, as Aben Ezra thinks, anyone that was deflowered by his father. Jarchi interprets it of his father's brother's wife, which he was obliged to marry by virtue of the law in (Deuteronomy 25:5)

nor discover his father's skirt ;
or lie with her his father had thrown his skirt over, or married; and which being the first, is mentioned here as a sample to all the rest forbidden (Leviticus 18:7) or, as Bishop Patrick expresses it, is a "short memorandum", to make them careful to observe all the other laws respecting incestuous marriages and copulations there delivered.