Is it permissible for a wife to shake hands with her husband's foster father, and for a husband to shake hands with his wife's foster mother?

It is permissible for the wife to shake hands with her husband's foster-father, and for the husband to shake hands with his wife's foster-mother, because the majority of jurists hold that the prohibition of marriage by affinity is established through fosterage. Ibn Taymiyyah disagreed with this, but the opinion of the majority is stronger and more cautious.