Your granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.
Grandaughter
Grandaughter.
If you are female and have a son, your aunt's niece's son could be you. Your aunt's niece's son could also be your first cousin, once removed, if he is the son of one of your first cousins. Your aunt also may have nieces who are not related to you, and then the son is not related to you either.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
Because her son passed away in 2000.
You would be second cousins.
Your niece's son is your grand nephew, and you are his great aunt.
grandaughter in-law
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
They are your nieces and nephews.
They are not related.