Is the marriage of my foster brother to my foster sister valid, knowing that the foster sister is the daughter of the maternal uncle?

There is no impediment for your cousin to marry your sister, as long as he is not her brother through fosterage (meaning that one woman breastfed both of them five known feedings within the two-year period), or if one of them is the maternal uncle or paternal uncle of the other, for fosterage prohibits what lineage prohibits. Your being a foster brother to his sister does not have any effect.