Step-brothers and step-sisters are a clear relationship, but what if the parents get divorced but the kids still keep in touch? Do the kids remain step-siblings?
What if the parents live together but never marry? And what about their other relatives? What should kids call the parents of their mother or father's domestic partner? Grandpartners?
Changes in families are difficult enough for children without the need to figure out terminology for relationships. It's too bad they must deal with things like that at all, but it might make their lives a tiny bit easier if we could come up with appropriate vocabulary.