what do you mean by "incestuous"? I'm having a hard time picturing the tree structure. Like if we keep things simple and assume every person on the tree "asexually spawns" multiple students, where does the "incest" come in?
Maybe one student having more than one advisor? If that's the case, usually it's just a thesis committee or reviewer or something, and not really multiple _main_ advisors
The academia is far more diverse than people think. More diverse between countries, between universities, between disciplines, and over time.
Having a junior and a senior advisor is fairly common. Sometimes the work is done in multiple institutions, with a separate advisor in each. In some systems, most people who supervise students are not formally qualified to do so, creating a need for a separate formal supervisor. Sometimes there are two equal advisors, and sometimes the advisor changes for various reasons. Sometimes the student is an independent scholar and the advisors are only loosely involved in the work. If you only have written records, it can be impossible to tell which of these was the case for a particular student.
To give you an example, about half of my group at University X did a Bachelor's in statistics at University X, then a Mater's in statistics at University X, then a PhD in statistics at University X, and some are even doing a PostDoc (in statistics at University X)!
but that's not what the original comment mentioning "incestuous" was talking about. The original was talking about academic trees, for which "incestuous" would indicate a tree with loops (i.e. multiple paths between two points)
Maybe one student having more than one advisor? If that's the case, usually it's just a thesis committee or reviewer or something, and not really multiple _main_ advisors