Praise be to Allaah.
It is haraam for a man to
marry the wife of his son as soon as the son does the marriage contract with
her, because Allaah says (interpretation of the meaning):
“Forbidden to you (for
marriage) are: … the wives of your sons who (spring) from your own loins”
He may marry the mother of
his son's wife, or her sister or her daughter - from a husband other than
his son - because Allaah says, after mentioning the women who are forbidden
for marriage, (interpretation of the meaning):
“All others are lawful,
provided you seek (them in marriage) with Mahr (bridal-money given by the
husband to his wife at the time of marriage) from your property, desiring
chastity, not committing illegal sexual intercourse”
So there is nothing in
sharee’ah to prevent the father marrying the sister of his son's wife.
And Allaah knows best.