Your grandchildren are your sister’s great-nieces or great-nephews.
In simpler terms, your grandchildren are the children of your children, and to your sister, they are her great-nieces or great-nephews.
Your grandchildren are related to your sister through a family tree.
The Family Tree [Explained]
You and your sister belong to the same generation, which is the first generation.
Your children, who are your sister’s nieces or nephews, belong to the second generation.
When your children have their own children, those children become the third generation in your family.
So, your grandchildren are part of the third generation in your family.
To your sister, these third-generation children are her great-nieces or great-nephews.
Long Story Short
In summary, both you and your sister are in the first generation. Your children are in the second generation.
Your grandchildren are in the third generation, making them your sister’s great-nieces or great-nephews.
What Are My Sister’s Grandchildren to Me?
Your sister’s grandchildren are your great-nieces or great-nephews.
Your sister’s grandchildren are in the third generation of your family tree.
You are in the same generation as your sister, which is the first generation.
Your children and your sister’s children belong to the second generation.
When your sister’s children have children of their own, those children become the third generation, and that’s where your sister’s grandchildren fit in.
To you, these third-generation children are your great-nieces or great-nephews.
Hope this helps.