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, because of their father, Bob. Similarly, Grace and Frank share their mother, Carol.

Erin and Grace have no parents in common, and are thus not half-siblings. However, is there a term for their relation?