Is it permissible for my wife, who wears the niqab, to show her face in front of my brother and my sister’s husband inside the house? And does that affect the reward for wearing the niqab?

A woman must cover her entire body, including her face and hands, in front of all non-mahram men, including her husband's brother and her sister's husband. This ruling is even more emphasized for those who frequently visit the home. Uqbah ibn Amir (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said: "Beware of entering upon women." A man then asked: "What about the hamw (in-law)?" He replied: "The hamw is death." The hamw refers to the husband's relative whom she is not forbidden to marry after he separates from his wife. If a woman uncovers her face or hands in front of these men, she disobeys her Lord and exposes herself to temptation.