Your neice's children are your grandneices and grandnephews. You are their great aunt or great uncle.
Your niece's children are your greatnephews or grandnephews and greatnieces or grandnieces. You are their great uncle or great aunt.
Great-niece.
Second Cousin
Nephews and nieces
your 2nd niece...!
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
They would be great nieces or great nephews.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
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.
Your mother's nieces' children are your mother's grand nephews or grand nieces. They are your first cousins, once removed.
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Uncle if you are male and aunt if you are female.