Question: Is it permissible for me to shake the hand of my wife’s sister, and to be alone with her or other than that? [It’s just] because I know [while I am married to her sister, then] she is prohibited to me.
Response: It is not permissible for you to shake the hand of your wife’s sister, and nor to be alone with her. That is because she is not from amongst those women who are [permanently] prohibited to you, rather, her case is the same as any other unrelated woman [for whom you are not a mahram]. So beware of leniency and procrastinating in this [matter], and may Allaah grant us and you success in all [that is] good.