my younger brother
Humble term used to refer to one's own younger brother when speaking to someone outside the family. Shows modesty about one's own family member.
My younger brother is in your care.
foolish younger brother
Literally 'foolish younger brother', but in practice this is the same humble usage as sense 1; the 'foolish' nuance is part of the self-deprecating humble expression rather than a separate meaning. Rarely used to actually call one's brother foolish.
My younger brother is still a student.
I apologize for the trouble caused by my foolish younger brother's oversight.