the concerned person divorces your second daughter while her marriage was not consummated, it will be permissible for him to marry her sister i.e. his nephew's mother. Likewise, it is permissible for that divorced daughter to marry the younger brother of her previous husband. It is worth mentioning that no `Iddah (woman's prescribed waiting period after divorce or widowhood) is to be followed by women who are divorced before the consummation of marriage for Allah (Exalted be He) says,
O you who believe! When you marry believing women, and then divorce them before you have sexual intercourse with them, no ‘Iddah [divorce prescribed period, see (V.65:4)] have you to count in respect of them.
Consequently, it is permissible for each one of the two brothers to marry whomever he desires to marry amongst your daughters. (Part No. 18; Page No. 230) However, concluding the contract of marriage of your widowed daughter is only permissible after the expiration of her `Iddah. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.