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

Can a man marry the sister of his son's wife?
Praise be to Allah.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 Allah says (interpretation of the meaning):
“Forbidden to you (for marriage) are: … the wives of your sons who (spring) from your own loins” [al-Nisa’ 4:23]. 
He may marry the mother of his son's wife, or her sister or her daughter - from a husband other than his son - because Allah 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” [al-Nisa’ 4:24]. 
So there is nothing in sharee’ah to prevent the father marrying the sister of his son's wife. 
And Allah knows best.