Yes, you can add a category under a forum.
A sibling is at the same level in the same parent node.
A child is as the name suggests, a child of the parent, the next level down.
- Parent A (sibling of Parent B)
- Child 1 of Parent A (sibling of Child 2)
- Child 2 of Parent A (sibling of Child 1)
- Parent B (sibling of Parent A)
- Child 1 of Parent B