Is it permissible for a widow to breastfeed her martyred brother's daughter—who lives with her—so that she becomes a foster sister to her son, and thus a mahram to him?

For fosterage to establish حرمة (prohibition of marriage), two conditions are stipulated: first, it must occur within the first two years of the child's life, and second, there must be five or more nursing sessions. The evidence for this is the Prophet Muhammad’s (peace be upon him) saying: (Fosterage does not prohibit except that which tears the intestines from the breast and occurs before weaning). Accordingly, the nursing of an adult does not have any effect and does not establish prohibition, according to the majority of scholars. Therefore, if the girl has reached six years of age, nursing her does not establish محرمية (the state of being an unmarriageable kin), because the nursing would then occur after the first two years. You must, when she grows up, have her observe hijab from your son, because he is not a محرم (unmarriageable kin) to her.