The great grandchildren of your great grandmother are:
Your great grandmother's step-father's cousin's are not related to you.
Your grandmother's brother is your great uncle. Your great uncle's grandchildren are your second cousins.
The direct descendents of a person are that person's children, grandchildren, great-grandchildren, etc. The direct descendents can trace their ancestry directly back to the person. For example, you are your great-great-grandmother's direct descendent, but you are not the direct descendent of your great-great-grandmother's sister.
Your great grandmother's children's children are her grandchildren. They could be any of the following:your grandparentyour second cousin twice removedyour great aunt or great uncle
Second cousins.
Not yet, but her eldest grandson is recently-married.
She would be their Great Aunt. Regarding immediate family, for every generation back you go, you add one great. For example; A great grandmother would be the grandparent's mother. A great grandmother's brother would be a great-great uncle.
Some of your great grandmother's great grandchildren are your second cousins. Some may be your first cousins and some could be your siblings.If they are descended from your great grandmother but not from your grandparent, they are second cousins. If they are descended from your great grandmother and your grand parent, but not from your parent, they are your first cousins. If they are descended from your great grandmother, your grandparent, and from your parent, they are your siblings (your brothers and sisters).
The grandchildren of your grandmother and the grandchildren of any of her siblings (including of her brother) are second cousins to each other.
If your friend's great grandmother was the sister of your great grandfather, then you are third cousins. The children of siblings (brothers and sisters) are first cousins. The grandchildren of siblings are second cousins. The great grandchildren of siblings are third cousins.
Your grandmother's sister is your great aunt. Your great aunt's daughter is your first cousin, once removed.
If you are referring to YOUR grandmother and grandfather then your children are their great grandchildren. If you are referring to your parents, then your children are their grandchildren. The primary kinship dominates.