What is the sin incurred of one's wife having illicit sex?
As per laws of Manu, V, l66. WOMEN. 197:
But a woman who from a desire to have offspring violates her duty towards her (deceased) husband, brings on herself disgrace in this world, and loses her place with her husband (in heaven).
Offspring begotten by another man is here not (considered lawful), nor (does offspring begotten) on another man's wife (belong to the begetter), nor is a second husband anywhere prescribed for virtuous women.
She who cohabits with a man of higher caste, forsaking her own husband who belongs to a lower one, will become contemptible in this world, and is called a remarried woman (paraptarva).
By violating her duty towards her husband, a wife is disgraced in this world, (after death) she enters the womb of a jackal, and is tormented by diseases (the punishment of) her sin.
In above verses bold part is highlighting the consequences of conducting such sins.