In the English language relationship words make no distinction between younger or older siblings.
Your mother's brother, whether younger or older, is your uncle and his daughter is your first cousin. His daughter is your mother's niece.
Your nephew?
your second cousin- in law or by her name lol
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
Niece
A niece
A niece
natini
Your niece.
A cousin.
We call "Mami" to moms brother's wife.
Its would be your neice
Niece