Although there are explicit sibling connections in Scabard, we recommend letting Scabard find them by setting "father of", "mother of" connections.
Imagine a situation like the Weasley Family in Harry Potter: six brothers and a sister. That would be 21 sibling connections. Cousins in a large family would be exponentially worse. Think of how easy it would be to forget some of the connections. It's actually less work to have bogus parents and let Scabard determine siblings and cousins.
Now imagine a case where you decided that two characters were siblings, but later forgot and gave them different parents. Now you have an inconsistency problem.
As mentioned above, siblings and half-siblings are calculated automatically as part of a Basic subscription, which walks up to the parent(s) and down to other children. But there is also the Advanced Connections for other types of more complex relationships (grandparents, aunts and uncles, sister-in-laws, etc).
The code that calculates more distant relatives uses "father of", "mother of" and "spouse of" and would be tricky to maintain if we had to consider siblings and half-siblings too.