Are my daughter's cousin and her maternal aunt's daughter considered siblings through rada'a (milk-kinship) because they were all breastfed by me during the same period?

It is not permissible for your daughter's male cousin to marry her maternal cousin, because they have become siblings through fosterage if they both nursed from you five known feedings within the period of fosterage. For the Prophet, peace and blessings be upon him, said: "Indeed, fosterage prohibits what consanguinity prohibits." And marriage is forbidden between siblings from the same mother by blood relation; likewise, it is forbidden between siblings from the same mother by fosterage. There is no difference in this regard whether you nursed them during one period or separate periods. For everyone who nurses from a woman with valid fosterage, she becomes his mother, and her children, whether by fosterage or consanguinity, become his siblings, regardless of whether he nursed with them, before them, or after them. What some people believe, that the prohibition applies only to those who nursed with him or after him, is an error that must be clarified.