Is it permissible for the stepmother to breastfeed the daughters of her deceased husband so that their maternal uncles by رضاعة (radha'a - fosterage) become mahram to the husband's daughters?

Prohibitory suckling, according to the majority of scholars, is that which occurs within the first two years of life. If the child exceeds two years of age, suckling does not establish prohibition. This is the view of most scholars, including Umar, Ali, Ibn Umar, Ibn Masud, Ibn Abbas, Abu Hurairah, and the wives of the Prophet (peace be upon him) except Aisha, as well as Al-Sha'bi, Ibn Shabramah, Al-Awza'i, Al-Shafi'i, Ishaq, Abu Yusuf, Muhammad, Abu Thawr, and one narration from Malik. Abu Hanifa disagreed, stating that suckling establishes prohibition within thirty months. Therefore, your wife suckling your daughters who have exceeded two years of age does not prohibit them from marrying their brothers or others.