Use their names. English does not have a word for this relationship.
That would be yourself
Your brother's daughters are your nieces
Your brother's daughters are your nieces
daughters-in-law
THEY DONT SELL THEIR DAUGHTERS
The possessive form of the plural, compound noun daughters-in-law is daughters-in-law's. example: Both of my daughters-in-law's birthdays are on the same day.
She is your niece.
Granddaughter
Grandson :)
You call your daughter's baby, your grand-duaghter.
You would call your sister's daughter your niece.
Nieces.