Is it permissible to marry the daughter of a paternal aunt if the questioner's paternal aunt breastfed all the children of his paternal uncle (the father of his current wife), meaning that this paternal aunt's daughter and his current wife are milk-sisters?

If your aunt breastfed your wife five full, satisfying feedings, your wife becomes a foster-sister to all your aunt's children. Therefore, it is not permissible for you to marry your aforementioned cousin as long as your cousin is your wife, due to the Almighty's saying: ﴿And that you take [in marriage] two sisters simultaneously﴾ [An-Nisa:23], and due to the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by fosterage."