The word tree[1] has a specific meaning when talking about computer science (recursion). An idealised biological family tree is a DAG (directed acyclic graph[2]), because there are cross linkages creating common ancestors, although I am also presuming we define when to stop adding nodes.

An actual family tree is not even a DAG because the nodes are unknown, and “parents” is an extremely fuzzy concept. Where does adoption fit it? What about step-parents? How do you record uncertainty? Etcetera.

One of my friends in another country had a kid with her first cousin, and parents can be more closely related biologically.

It is less than one hundred generations before we find a single common ancestor to everyone alive[3]. Also see [4][5].

Regardless, anyone that thinks using a family tree as a teaching example for children should have a reëducation holiday </unfunny-black-humour>.

[1] https://en.wikipedia.org/wiki/Tree_(data_structure)

[2] https://en.wikipedia.org/wiki/Directed_acyclic_graph

[3] https://www.scientificamerican.com/article/humans-are-all-mo...

[4] https://en.wikipedia.org/wiki/Mitochondrial_Eve

[5] https://en.wikipedia.org/wiki/Y-chromosomal_Adam

Eventually your (for example) great*N paternal grandfather is going to show up on your mother's side, forming a loop. That's just due to bounds on the human population and how the number of labels for previous generations grows exponentially backwards in time.