Does the questioner's not looking at girls in the street, for fear that someone might look at his sister, prevent someone from looking at his sister if her brother looks at girls?

Since gazing leads to the corruption of the heart, Allah has commanded the guarding of the sight and the private parts. Allah Almighty says: (Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.) The Prophet, peace and blessings be upon him, forbade following one gaze with another. Whoever refrains from this sin, seeking the pleasure of Allah, will receive a great reward, and Allah will replace it for him with an act of worship whose sweetness he will find, as the Prophet, peace and blessings be upon him, said: "There is no Muslim who looks at the beauty of a woman for the first time and then lowers his gaze, but Allah will replace it for him with an act of worship whose sweetness he will find." Lowering the gaze should be an act of obedience to Allah Almighty, not for the sake of the sister. If one purifies his intention, Allah may protect his sister as an honor to him. We hope that the other brother will be like him in refraining from looking at what is not lawful for him.