Is it permissible to mix a mother's milk with other milk to change its taste, and does this establish the ruling of رضاعة (wet nursing)?

It is permissible to breastfeed your brother's daughter, and to mix the milk with something that improves its taste or makes it acceptable to the infant. If the child nurses five feedings, she becomes your daughter through suckling. There is no harm in breastfeeding a child with the intention of establishing mahramiyya (prohibition of marriage due to kinship). Sheikh Ibn Baz was asked about a child taking medicine mixed with the milk of another woman, repeating this five or more times. He replied that suckling establishes prohibition under its legal condition, which is five known feedings or more within two years (of age). If milk is mixed with other things like medicines, it produces its known effect, and it is treated as if one has nursed five feedings or more. What is expressed for the child and given to him is equivalent to what he nurses from the woman's breast; thus, it carries the ruling of suckling if that is known.