If you've entered a person but can't see them in your family tree, you will need to change the "View Person" for the tree to that of one of that person's parents or grandparents. To do this, click the triangle in the lower corner of either parent's box, or that of a grandparent.  This turns that box gray, showing that it is the current "View Person", and shows their tree view.

The View Rule 

Only part of your tree can be seen at any given time, and a given person's tree view does not show their siblings or siblings of ancestors.

The tree view for any person shows a) both of their birth parents and direct grandparents, b) their spouse and any prior spouses, c) their descendants for three generations and all current and former spouse descendants for three generations.

Reasoning

This app is focused on using the family tree to easily navigate to the collections of those for whom you most frequently view photos, videos and other memories.  We reason that a given grown person will most often view the collections relating to their own childhood family and their current family, including all of their offspring.  Having siblings on the tree in this view might make it unwieldy. 

If one wants their or an ancestor's siblings shown on the tree, they can select one of the parents of this person as the View Person by clicking the triangle in one of the parent's boxes, and all children and three generations of descendants of the parent selected will be displayed.  The further up the tree the View Person, the more people in general will be seen on your tree at one time.