What is the ruling of the Sharia regarding a man marrying his maternal uncle's daughter, knowing that their deceased grandmother had breastfed the wife, and what is the ruling regarding a man marrying his maternal aunt's daughter, knowing that his living grandmother breastfed the wife's sister and that the husband's mother also breastfed the wife's sister, and is it permissible to avoid marriage within this tribe for fear of doubt?

If your grandmother breastfed your خال's daughter five full feedings within two years, then your خال's daughter becomes your maternal aunt through breastfeeding. The breastfeeding of your wife's sister by your grandmother or mother does not affect your marriage to her sister, because the prohibited relationship is with someone who breastfed from you, or from your mother, or your grandmother. So, whoever breastfed from your grandmother is a maternal aunt to you, and whoever breastfed from your mother is a sister to you. There is no doubt that would prevent marriage to the sister of someone who breastfed from your grandmother or mother.