Your great aunt's daughter's children (her grandchildren) are your second cousins.
They would be great nieces or great nephews.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
Your great grandparents brothers and sisters are your great uncles and great aunts.
I believe they would be great aunts or great uncles.
great great (etc.) grandparents great grandparents grandparents + great uncles and great aunts parents + uncles and aunts [=parents' siblings] siblings + cousins [=uncles' and aunts' children] children + nephews and nieces [=siblings' children] grandchildren great gandchildren great great (etc.) grandhildren also cousins and second cousins [=(great-)grandparents' descendants]
great great (etc.) grandparents great grandparents grandparents + great uncles and great aunts parents + uncles and aunts [=parents' siblings] siblings + cousins [=uncles' and aunts' children] children + nephews and nieces [=siblings' children] grandchildren great gandchildren great great (etc.) grandhildren also cousins and second cousins [=(great-)grandparents' descendants]
I believe they would be great aunts or great uncles.
your cousin
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
your nan's sister is one of your parent's aunts. therefore, she's your great aunt x
The possessive form for the plural noun great aunts is great aunts'.Example: My three great aunts' house is a great place to visit.
Your Aunt is your children's Great Aunt.