I assume you know that these marriages were illegal from 1835 until the Marriage (Prohibited Degrees of Relationship) Acts, 1907 and 1921, that is the Deceased Wife's Sister Marriage Act, 1907 and the Deceased Brother's Widow's Marriage Act, 1921. However many couples ignored the law, and many clergy overlooked it.
One difficulty is how would the person carring out the wedding know that the proposed marriage partner was the deceased wife's sister, or the deceased brother's widow?
Stan