Are the sister and her husband considered foster parents to her brother’s daughter, whom she breastfed intermittently during her first two years, and is her brother’s daughter considered a foster sister to the sister’s children, and must she wear the hijab in front of them?

If a sister breastfeeds her brother's daughter five times during the nursing period, then she becomes her foster mother, and her husband becomes her foster father, and their children become her foster siblings. The brother's daughter is permitted to appear before her sister's husband and his male children without a hijab (definition), because they are her mahrams, based on the Prophet's (peace be upon him) saying: "Fosterage prohibits what birth prohibits." This ruling applies only to the one who was nursed, excluding her siblings who were not nursed. A woman must adhere to the limits of interaction with her mahrams.