Doing some experimenting, I found the following:
When selecting myself:
I can see myself + spouse & our children
I can see each of my siblings + spouses & their children
I can see both of my parents, each of their siblings + spouses (my aunts & uncles) & their children (my first cousins) and grandchildren (my first cousins once removed)
However, I don't see my spouse's parents or siblings. Also, going back beyond my parents, I can only see direct ancestors: my mother's & father's parents (my grandparents), but none of their siblings (great aunts & uncles). Same for their parents and so on; only direct ancestors.
When selecting my father's father (my grandfather):
I can see all of his direct descendants (sons & daughters, grandchildren, great-grandchildren) and each of their spouses, but none of the spouses' ancestors, e.g. not my mother's parents or her siblings, nor my siblings' spouses' parents or siblings.
I can see his (my grandfather's) siblings and each of their descendants and their spouses, but again no additional family for the spouses.
So the rule seems to be that you can see the selected person's immediate family, including parents/aunts/uncles/cousins, and their spouses, and that person's direct ancestors, but nothing more of the spouses (parents or siblings) or siblings of grandparents or beyond.
If you are trying to get a "complete" family tree beyond 2 generations, the Family Echo GUI doesn't appear to provide a means to do that. And realistically, such a diagram would quickly become unwieldy.