Most people are more interested in ancestors than their siblings, but records of the siblings can contain vital clues.
For example, death duties files (memories van successie) of siblings who died without children can show that nieces and nephews or more distant relatives inherited. The death duties files can reveal complex family relationships, especially in the case of an elderly person who died after their siblings.
Another tip is to always check the marriage supplements of marriages of siblings, especially the younger siblings. If they married after their parents had died, the supplements may contain extracts of their parents’ death records and even of the grandparents.