Is it permissible for a man to marry his sick wife's sister—she having been afflicted with paralysis for five years and having lost her mind, movement, and speech—either without divorcing the first wife, or after divorcing her at the request of the wife's family on the pretext that her sister will care for her?

It is not permissible for a man to combine two sisters in marriage, and this is forbidden by consensus, based on the Almighty’s saying: (And that you take two sisters as wives). If a man divorces his wife and her waiting period (iddah) has ended, he is permitted to marry her sister. He is not obligated to inform his wife of this, especially if she has lost her sanity.