Question 14: There is a woman who has a married daughter. This woman veils herself before the husband of her daughter; she does not eat with him; and she does not shake hands with him even on occasions. What is the ruling on this?
Answer: A son-in-law is a Mahram (permanently unmarriageable relative) for his mother-in-law. Allaah (Glorified be He) says about unmarriageable relatives from among women: …your wives’ mothers This matter is unanimously agreed upon among scholars… read more here.