Wiktionary
alt. The child of one's stepfather or stepmother from another relationship; a stepbrother or stepsister n. The child of one's stepfather or stepmother from another relationship; a stepbrother or stepsister
Wikipedia
A stepsibling or stepsib is the offspring of one's stepparent. This would constitute a stepsister (or stepsis) and a stepbrother (or stepbro).
Usage examples of "stepsibling".
Looking back at her stepsiblings, now headed in a wild melee toward the lake, she drew in a long breath, then, lips firming, turned back into the library.
Knowing her as he did, knowing how closely she'd been involved with the care of her stepsiblings, he at first found that odd.
She gestured to her stepsiblings, blissfully unaware of the clouds hovering on the family's horizon.
Which is how Kristy acquired her stepsiblings Karen and Andrew Brewer.
Kristy also acquired two younger stepsiblings, Karen, who is seven, and Andrew, who is four.