All Praise to Allah Swt and blessings on Prophet Muhammad peace be upon him His family and companions
Whether she is a widow or you get permission from your wife you are still not allowed to marry her in this situation.
Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.(Quran 4:23)
Narrated Um Habiba: I said, "O Allah's Messenger (ﷺ)! Do you like to
have (my sister) the daughter of Abu Sufyan?" The Prophet (ﷺ) said,
"What shall I do (with her)?" I said, "Marry her." He said, "Do you
like that?" I said, "(Yes), for even now I am not your only wife, so I
like that my sister should share you with me." He said, "She is not
lawful for me (to marry)." I said, "We have heard that you want to
marry." He said, "The daughter of Um Salama?" I said, "Yes." He said,
"Even if she were not my stepdaughter, she should be unlawful for me
to marry, for Thuwaiba suckled me and her father (Abu Salama). So you
should neither present your daughters, nor your sisters, to
Allah Swt and His Messenger knows best.