As “sibling” refers specifically to a brother or sister, there isn't a true antonym in the traditional sense. However, we can think about words that represent the absence of a sibling relationship in a way:
Only child: While not a direct antonym, an only child has no siblings. For example, “She grew up as an only child, longing for the companionship that siblings might have provided.” An only child doesn't experience the bond and rivalry that come with having a sibling.
Alone/Isolated: These words imply a lack of family connection in the context of siblings. For instance, “He felt alone without siblings to share his joys and sorrows.” When someone is without siblings, they may sometimes feel isolated in their family experience.
In conclusion, while there isn't a perfect antonym for “sibling,” words like “only child” and “alone/isolated” can help convey the absence or lack of the sibling relationship.