What is the ruling on shaking hands with a sister’s husband, and does shaking his hand, if it is permissible, invalidate the ablution?

Question:
What is the ruling on shaking hands with a sister’s husband? And does shaking his hand, if it is permissible, invalidate the ablution?

Answer:
Shaking hands with a sister’s husband is not permissible; because he is a non-mahram to her. The mere fact that he is her sister’s husband does not make him a mahram to her. It is not permissible for her to shake his hand, nor to shake the hand of any man who is not among her mahrams; because that is a cause of fitnah (trial, discord or temptation), and because the Prophet (peace and blessings be upon him) did not shake hands with women who were not among his mahrams (peace and blessings be upon him), and he is the role model.
As for whether a woman shaking hands invalidates the ablution, this is a matter of scholarly disagreement among the people of knowledge regarding touching a woman without a barrier. Some scholars hold that it does not invalidate the ablution in any case, some hold that it invalidates the ablution in any case, and some distinguish, saying that if it is for desire, it invalidates the ablution, and if it is not for desire, it does not invalidate the ablution. This opinion is likely the most just of the opinions and the closest to being correct. However, it should not be understood from the fact that it does not invalidate the ablution according to those who said so that it is permissible; rather, it is forbidden and the one who does it commits a sin. The discussion is only about whether his ablution is invalidated along with his sin, or whether it is not invalidated. Yes.