The mother of your uncle (by marriage) is not related to you. Neither are any of her other descendants or relations.
Your mothers sisters Granddaughter is your second cousin
Great neice
Your mothers sisters Granddaughter is your second cousin
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
Your sisters grandson would be your grandnephew.
Your granddaughter and your sister's grandson are second cousins.
Your grand-niece.
Yes
Your son and your sisters child are first cousins. The granddaughter is out of synch by a generation. That will make them first cousins once removed.
The relation of the mother;s sister to daughter is niece-aunts.
Your sister's granddaughter is your great niece and you are her great aunt (if you are a woman) or great uncle (if you are a man).
What family members your is fathers mothers daughters sons sisters