A person’s "tree view" does not show their siblings. To see siblings, click the triangle of a parent and go to their view. (Clicking the triangle in any node turns it gray and makes it the View Node, showing this person’s tree.)
The reason for this view rule is that it is generally assumed that a given grown person with a large family loves his/her siblings, but generally will be accessing the collections relating to their own childhood (likely stored in their parents' couples collection) and that of their offspring. Having siblings on the tree in this view might make it unweildy.
If one wants a view with their siblings shown on the tree, they can select a parent as the View Node (or even Home Node) and all siblings entered will be displayed, along with any offspring.