Nephews and nieces
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.
You're not related.
granduncle - grandnephews/grandnieces
Uncle if you are male and aunt if you are female.
Your niece's children are your great nieces and great nephews.
There is 2 answers your mothers brothers children or your dads sister;"s children
Your great nieces or newfews
Great-niece.
Second Cousin
your 2nd niece...!