Hello Elsa,
Uschi told me that the explanation which I gave to you was wrong.
In Franconia "Geschwisterkind" normally does not mean brother/sister, but
cousin.
Therefore,
"von Vatterseiden herr leibl. Geschwister kind mit mir" would mean:
"The person is the child of a brother/sister of my father".
Sorry for my mistake, and thanks to Uschi for putting it right.
Karl