Is it permissible to marry the sister of my brother's wife, given that my mother claims to have breastfed my brother's wife, but there are no witnesses to this, and knowing my mother's frequent dishonesty?

Many scholars have held that the testimony of a single woman is acceptable regarding suckling (radāʿ). This is based on the hadith (definition) of Uqbah ibn al-Harith: "A dark-skinned maidservant came and said, 'I suckled both of you.' So I came to the Prophet (peace be upon him) and mentioned that to him. He said, 'How so, when she claimed that she suckled both of you?' and he forbade him from her." Therefore, the mother should be reminded of the seriousness of this testimony and its implications. If she insists, then the marriage is not permissible. If she retracts, then it is permissible. And if the mother dislikes the marriage to that girl, then she should be left alone, for it is not an act of filial piety for the son to marry a girl whom the mother does not wish to be with him.