I have a paternal uncle who is married to two wives, one of whom breastfed me. Is it possible for me to marry the daughters of the other wife?.
If you were breastfed five times in the first two years of life by your uncle’s wife, then she is your mother through breastfeeding and your uncle is your father, and all his sons and daughters from this wife and from all his wives are brothers and sisters to you.
Based on that, the daughters you asked about, if they are your uncle’s daughters, are mahrams to you; if they were from another husband, someone other than your uncle, then there is nothing wrong with you marrying one of them.
Shaykh Ibn Baaz (may Allaah have mercy on him) said: If a woman breastfeeds a child five times within the first two years, or more than five times, the infant becomes a son to her and her husband, who is the “owner of the milk”, and all the children of the woman from the husband who is “the owner of the milk” or another husband become brothers and sisters of the infant, and the children of the husband who is the “owner of the milk”, from the wife who breastfeeds and other wives, become brothers and sisters of the infant who is nursed. End quote from Majmoo’ Fataawa Ibn Baaz (22/274).
He was also asked (may Allaah have mercy on him): I was breastfed by a woman, then her husband took another wife and that wife had children; are they brothers to me?
He replied: If you were breastfed five or more times and the milk was attributed to the husband because it resulted from a pregnancy that occurred due to him, then they are your brothers through your father and your mother through breastfeeding. As for the children from his second wife, they are brothers to you through your father through breastfeeding. End quote from Fataawa al-Shaykh Ibn Baaz (22/305).
And Allaah knows best.