Is it forbidden for sisters to appear before the sons of their maternal aunt if the eldest sister was nursed by the maternal aunt's mother, and is it permissible for them to appear before the sons of their maternal uncle if the maternal uncle was nursed with the eldest sister when she was of nursing age and he was an infant?

Regarding the first issue: If the eldest sister was breastfed by her aunt's mother five full breastfeedings before the age of two, she becomes a sister to the aunt through breastfeeding (radha'a), and the aunt's sons become mahrams to her. However, the rest of the sisters are not sisters to the aunt, and her sons are not mahrams to them.

Regarding the second issue: If the wet nurse is the mother of the questioner's sister and the mother of her other sister, then the maternal uncle becomes a brother to both of them through breastfeeding, and it is permissible for them to appear before his sons. If the wet nurse is the stepmother of the questioner's sister (the mother of her other sister), then the maternal uncle is also a brother to both of them. However, if the wet nurse is a foreign woman, then the maternal uncle is a brother only to the girl who was breastfed with him, and it is permissible for her to appear before his sons, but not for her other sisters.

The conditions for breastfeeding to establish prohibition (tahrim) are that it must occur before the child reaches two years of age, and consist of five full breastfeedings.