Can a mother-in-law have an affair with son-in-law?

It is possible, but not very common.

Answer

May I expand on the first answer? As was said it is possible, so I ask why is it possible. The father may be a bad lover and is unable to satisfy his wife. The son and his mother in law may be attracted to one another sexually but not be in love. She can still love her husband but is sexually satisfied by his son. Such an act if the father found out would be grounds for divorce, and the son banished from the home.