I was a den leader for five years. I assume it varies by Pack or even Den, but in our Den/Pack the Den meetings were for the Scouts and perhaps a parent only. In the early years I liked to have a parent or two present to help with behavorial issues. As they got older I felt more comfortable having them on my own. I would NOT have wanted any siblings at the Den meetings or anywhere we were working on a specific requirement because it would have been too distracting. It was also difficult as it was to control the scouts sometimes, much less their siblings.
However, our monthy Pack meetings were very family-friendly and often included the entire family. Our Pinewood Derby encouraged family members to participate and even had an "Open Class" race for siblings and parents who made a car. My non-Scout son participated in that several times and had a blast making a car with his dad and brother.
For camping trips it was generally the Scout and one parent. I don't think siblings were allowed on camping trips. The ony times I knew multiple parents to attend were when all of their children were involved in Scouting.
It might depend on the age and activity level of your other children. We did have younger siblings at Pack meetings who could be disruptive and distracting. They were never told they couldn't attend, but they sometimes distracted from the activity/entertainment or their parents had to take them out into the hallway. It does seem strange he told you siblings couldn't attend the Pinewood Derby when there is a sibling race.