I just had dinner with some family.
There, was the mother of my father (grandmother), the brother of my grandmother (grand uncle?), his daughters (???), and their children (???)
How do you call the daughters of my grand uncle and how do you call their children?
The mother of your father ... your grandmother.
Her brother ... the uncle of your father ... your great-uncle.
His daughter ... your father's first cousin ... your first cousin once removed.
Her children ... your second cousins.
See the chart here.
Kinship terms in English begin with the first common direct ancestors. People who have grandparents in common are first cousins; great grandparents, second cousins, and so on.
It gets a bit tricky when people are of different generations. If Person A is the great grandparent of Person B but the grandparent of Person C, then B and C are first cousins once removed, i.e. the difference between a great grandparent and a grandparent is one generation. So your parents’ first cousins are your first cousins once removed. In other languages, such as German, these are called aunts and uncles as well.
To look at it another way, your grandparents’ siblings are your great uncles or great aunts — more rarely, grand uncle/aunt. Their children are your parent’s first cousins and your first cousins once removed. You share common great grandparents with the children of your parent’s first cousins, so they are your second cousins.
You may find this chart helpful.
Source: Wikipedia.
