Is it permissible to pray with the wife's sister in solitude, and is it permissible to pray with the wife and her sister together?

It is not permissible for a man to be alone with a non-mahram woman, even during prayer, due to the saying of the Prophet, peace and blessings be upon him: "No man should be alone with a woman, and no woman should travel except with a mahram." If the seclusion is absent, then it is permissible.