Author has incestual relationship in their family. At least in my country that is highly offensive. For the sake of their possible children / siblinks, I hope they use protection!
Now I want to know in which culture this is used as a common insult.
The english "mother fucker" is born of the original taboo of fucking one's mother. The act is seen as bad, so we use the phrase referencing the act as a label for all sorts of bad things. Like, you dropped a book on your foot, and that's as bad as someone who sleeps with their mum, so you say "mother fucker".
Spanish has tu chinga tu madre or just chinga tu madre.
Not sure off the top of my head, but I know there's lots of other cultures that use the same taboo in the same kind of substitional way.