The child of your grand niece (also called great niece) is your great grand niece or nephew (or great great niece or nephew). If you are a woman, you are that child's great great aunt; if you are a man you are the child's great great uncle.
I believe they would be great aunts or great uncles.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
2nd Cousin
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]
The grandchildren of any of your great uncles and great aunts are your second cousins.
Your aunts' grandkids are your cousins. They are the children of your aunts' children, making them part of your extended family. Depending on the number of aunts and their children, you may have multiple cousins who are considered your aunts' grandkids.
The possessive form for the plural noun great aunts is great aunts'.Example: My three great aunts' house is a great place to visit.
There are Aunts, great aunts, great-great aunts, et al. A second aunt would be if you have two aunts and you prefer one over the other. But no there isn't.
The cousins of your in-laws are not related to you and neither are their parents. However, the parents of your spouse's first cousins are your spouse's aunts and uncles, and thus your children's great aunts and great uncles.
One of the children of your great great great great grandmother is your great great great grandmother or your great great great grandfather. The others are your great great great great aunts and great great great great uncles.