He was breastfed by the wife of his paternal uncle; are the daughters of his paternal uncle from his other wife forbidden to him for marriage?

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.