This is probably best explained with an example. Alice and Bob have a child, Erin. Bob and Carol have a child, Frank. Carol and Dan have a child, Grace. Clearly, Erin and Frank are half-siblings, ...
I had an uncle (my father's brother) who I never met because he died before I was born. He has 2 kids though, my first cousins. His widow has since re-married and had 2 more children. Is she still my ...