What Do You Call Your Child's Sibling

Pretty much in the title but to make it more explicit:

• You have a child X
• X has a half-sibling Y
• You are not in a relationship with either of Y's parents (formerly you were with one of them to produce X)

What do you call Y with respect to yourself?

There is no standard word for this relationship. Kinship is not usually treated as transitive -- your cousins may have cousins with whom you share no ancestor, and thus you are not usually considered related to such cousins' cousins.

"My ex's kid" is probably an easily-understood shorthand. It does not sound as warm as might be ideal, but I suspect "my daughter's (half-)sister" would prove more distracting than helpful.

If you are bound to this child by care or affection, a word like "niece" or "step-daughter" might be employed, though such terms might be misunderstood as presumptuous as well. The idea of a niece/aunt relations is quite traditionally very flexible, and used outside of its literal meaning.

There is no term for such a relationship. For all intents and purposes, you and Y are not related at all. For below, Foo is you, Bar is the other parent of X, and Qwerty is the remaining parent of Y.

• X is Foo's child.

• Y and Foo have no relation.

• X and Y are both children to Bar.

• X is a stepchild to Qwerty (only if Bar and Qwerty are married).

• Y is X's half brother or half sister.

Hope this helps.