This is what you wrote: “Stepsiblings are also, in the grand sweep of history, a relatively new phenomenon.” Perhaps it would have been better to write what you just did, rather than this very grand, sweeping, not terribly accurate sentence. Also, step-siblings has a hyphen in it.

How do you figure step-siblings are a recent phenomenon? It used to be that people died, early and often, leaving their young spouses with children, who then remarried. Step-children have been around since, well, forever.