Is it permissible for the wife of the questioner to uncover herself before his foster brother after the questioner was suckled by the foster brother's wife with the prescribed number of sucklings?

Your foster brother is a forbidden (mahram) relative to your wife because she is the wife of his foster son. She is forbidden to him just as his biological son's wife would be, based on the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by fosterage."