Because dragons are so long-lived and never experience infertility as a result of aging, dragons often have siblings with multiple generation gaps between them. If they’re part of a clan where it’s typical for hatchlings to strike out on their own once they come of age, it’s possible for them to have several siblings decades younger or older that they haven’t even met. Even with the same parents, siblings several generations apart are more likely to have a relationship similar to second or third cousins, in that they’re extended family that they infrequently interact with.
Clutchmates are a different matter. Since they all hatch around the same time, it’s much easier for dragons to form a sibling-like bond with a hatchling from the same clutch of eggs. When making family trees, it is important to specify which dragons are clutchmates, as it is considered a distinct category from sibling and crucial to accurately charting generation lines. Also, if a clan raises hatchlings communally, this familial kinship can also extend to clutches from different parents if they hatch en masse. For this reason, it’s not uncommon for one dragon to refer to another clan member they grew up with as as brother or sister, despite having different parents.