Is it permissible for the questioner's sister to marry her mother's nephew, given that the questioner's maternal uncle (her mother's brother) was suckled five full feedings by her mother?

What is prohibited by lineage is also prohibited by fosterage. So, whoever is suckled by a woman becomes her child and the child of her husband, the owner of the milk. This applies only to the suckled child, not to their siblings. Therefore, it is permissible for your maternal uncle's son to marry your sister if your mother did not suckle him, and there was no other fosterage between them. The mere fact that your mother suckled her brother does not prohibit her other nephew from marrying into your family.