Cousins
They are your nieces and nephews and not your cousins
If you have two nieces who are not sisters but are each the daughters of your siblings, the two nieces are first cousins. If they each have a daughter, those daughters are second cousins to each other.
No, They are not cousins.
No, they are not cousins. They are not related.
Your mother's nieces' children are your mother's grand nephews or grand nieces. They are your first cousins, once removed.
They are 1st cousins once removed.
Second cousins.
They are 2nd cousins.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.