Can a man marry the sister of his son's wife?
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.