How can one deal with younger siblings who do not respect their older brother, are stubborn, sometimes use violence, and behave impolitely in front of others?

You should mend the relationship with your siblings by treating them kindly, overlooking their mistakes, being good to them, and avoiding quarrels. This will earn their respect and appreciation. Their lack of respect might be due to your manner of interaction or their young age. Gentleness adorns everything, and violence disfigures it. "He is not of us who does not show mercy to our young and respect our elders." Using gentleness, leniency, and forgiveness is better than arguing and cursing.