Is it permissible for a husband to shake hands with a married or grown-up sister-in-law being a temporary Mahram to her?
Is it permissible for him to go to his father-in-law's house and sit alone with his sister-in-law? It should be noted that he treats them exactly like sisters.
(Part No. 17; Page No. 422)
It is not permissible for the husband to shake hands with his sister-in-law because it is permissible for him to marry her if his wife dies or if he divorces her. Also it is not permissible for him to sit in Khulwah (being alone with a member of the opposite sex) with her. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.