And please how me where God commanded us to fast.
Jesus said that we shall fast (Mk 2:20. Mt 9:15) and fasted himself (Mt 4:2). St. Paul directed us to "exhibit ourselves as the ministers of God" by a variety of mortifications, including fasting (1 Cor. 6:4f). So, Catholic Christians fast.
It doesn’t say, “if your brother demands that you fast and you don’t think you should have to”
Well, what are you doing on this thread then? Your fellow Christians fast and urge others to fast and you apparently take offense. We have a dispute. We should ask the Church, which shall "bind and loose" in all matters (Mt 18:18). I have.