3rd cousin I think:-)
4 daughters, 3 sons.
Niece
Your daughter and your first cousin are first cousins, once removed.
your second nephew
Second cousins once removed.
First cousin once removed.
It says what relation are your brothers nieces to you.... It could be your child or your niece also causes you can have two brothers and it may be the other ones kids. Or of course it could be your brother's new wife kid by another man making her no relation.
Technically there is no relationship. When introducing her, you would refer to her as My daughter's mother-in-law.
If there are 6 daughters and each daughter has one brother, that means there are 6 brothers. 6 daughters and 6 brothers = 12 children by using addition.
your step niece,,, if it was your brothers own daughter it would be your niece, since its his stepdaughter, than its your step niece.
They would be cousins, once removed.