The children of siblings are first cousins... the grandchildren of siblings are second cousins, and
Die Kinder der
Geschwister sind Cousins 1. Grades, die Enkelkinder Cousins 2. Grades und so weiter.
Because it's obvious that there are things among siblings which are totally out of the question.
Denn für
Geschwister ist einiges verboten. Absolut verboten.