What is the ruling on Ammar marrying one of the questioner's daughters, knowing that the questioner's wife, Amina, breastfed one of his brother's sons (Anas or Ammar) once or thrice, and that the questioner's mother, Aisha, and his maternal aunt, Zaynab, confirmed that it was Anas who was breastfed once?

It is permissible for Ammar to marry your daughter if your mother and your maternal aunt (Zainab) testify that the one nursed by your maternal aunt (Amnah) was Anas and not Ammar. Their testimony is sufficient. In fact, testimony regarding fosterage is established by a single woman; the proof for this is the hadith of Uqbah bin Al-Harith in Sahih Al-Bukhari, where the Prophet (peace be upon him) separated him from his wife based on the testimony of one woman who had nursed them both, as mentioned by Ibn Qudamah in Al-Mughni.