Is it permissible for the questioner's sister to marry her maternal cousin's brother, knowing that the questioner's mother nursed her maternal cousin, and the questioner's paternal aunt nursed the questioner?

Your paternal aunt is your mother through radaah (suckling), and her children, brothers, and sisters whom she suckled have become your siblings through radaah. Similarly, your mother has become your paternal aunt's son's mother through radaah, and everyone she suckled has become his sibling through radaah. This is based on the Prophet, peace and blessings be upon him, who said: "Radaah prohibits what lineage prohibits." As for your brothers and sisters who were not suckled with their paternal aunt's children by your mother or another woman, they are not siblings to them, and it is permissible for one of your paternal aunt's children to marry one of them.