Okay lets go in code. You are person Z and your relation is person Y.
Sisters A and B have children who are first cousins to each other (one of these is Person Zs mum).
These first cousins have children who are second cousins to each other. One of these second cousins is Person Z. The other cousin has a child. This child is Person Y.
Person Y and Person Z are second cousins once removed (the once removed shows that there is new blood so to speak in the family).
If person Z (you) were to have children, they would be th
ird cousins with person Y.
Does that make sense?
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 grandchild and the sisters grandchild would be second cousins. The additional generation indicates they would be second cousins once removed.
Children of sisters are first cousins. Grandchildren of sisters are second cousins. Great grandshildren of sisters are third cousins.
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).
If your great grandmothers were sisters, then you are third cousins.
You would be second cousins.
You and your grandmother's great niece are second cousins.
Your child and your sister's child share a common grandparent and are first cousins. Your grandchild and your sister's grandchild share a common great grandparent and are second cousins. There is a generation out of synch, so they would be second cousins once removed.
If two people have great grandmothers who were sisters, the two people are third cousins.
Your great grand unlce's great grandniece could be you (if you are a girl), any of your sisters, or some of your first cousins or second cousins or third cousins who are girls.