It depends on what you mean.
If you're asking little brother to help you:
עזור, ××— קטן (azor, ach katan)
If you're asking a male to help a little brother:
עזור ×ת ××— קטן (azor et ach katan)
If you're asking a female to help a little brother:
עזרי ×ת ××— קטן (izri et ach katan)
akh katan (אח קטן), which literally means "little brother". ha-akh ha-tze'ir (האח הצעיר), which literally means "the youngest brother"
sister and brother = achot veh ach (×חות ו××—)
you can say "nong" to a little brother or sister
ha'ah hanotzri sheli
atah akhi? (אתה אחי)
"Brother of" and "brother" are written and pronounced the same: ach (אח) (ch is a gutteral sound)
Katan (קטן)
you can say "nong" to a little brother or sister
my big brother = achí ha gadól (אחי הגדול)
In Farsi, you say "برادر کوچک" (baradar kuchak) to refer to a little brother.
ktsat (קצת)
You would say "細佬" (sai3 lou2) in Cantonese to refer to a little brother.