[rhn_mk1]

A family tree is an overloaded term. The tree of ancestry, which describes who beget whom, sure cannot be cyclical.

The graph of civil relationships can certainly contain cycles. If the relationship in question is "is a legal parent of", for example, then adopt your parent.

The opportunities for cycles multiply if both blood and legal relationships are taken into account.

[lidHanteyk]

So don't mix those two things, then; keep them distinct. Work to preserve the acyclic property, rather than trying to toss new concepts in.

[rhn_mk1]

This works only until the concepts are the reason the problem is being tackled. A family tree based only on genetic relationships is interesting to few people outside geneticists and doctors. Most people are interested in familial ties as they are culturally defined.

[Retric]

For an actual example, Woody Allen’s marriage to his adopted daughter essentially resulted in her being her own mother.

[barrkel]

To be clear, Allen never adopted Soon-Yi Previn; she was adopted by Mia Farrow and Andre Previn before Farrow & Allen got together.

[Retric]

Yea, it’s also not the best example because Mia Farrow was not legally wed though Allen was in a long term relation and they had a child. So, I am not sure if it qualified as a common law marriage before their separation.

In an August 1992 interview with Time Magazine Allen said, “I am not Soon-Yi's father or stepfather”, adding, “I’ve never even lived with Mia. I've never in my entire life slept at Mia's apartment, and I never even used to go over there until my children came along seven years ago. I never had any family dinners over there. I was not a father to her adopted kids in any sense of the word.” Adding that Soon-Yi never treated him as a father figure and that he rarely spoke to her before the affair, Allen seemed to see few issues with their relationship.[196]. https://en.wikipedia.org/wiki/Woody_Allen

However, if your mapping out relationships in software without considering the timeline it looks like a circular relationship. Further, I am sure you can find cases that are actually incestuous.