why should it be ridiculous or not believable? if tomorrow we found out that all DIKs are cousins of various degrees it would be possible. the question is whether it is useful to the story, to the entertainment.
the kinship or close relationships between the various characters for what we have seen so far have been?
they have certainly been useful to DPC that in this way has created relationships between characters without having to explain them, without having to show us much (Derek and Maya love each other because they do, Josy and Tommy can not stand each other because it is so). for this I call it a cheap and lazy choice, the laziness on which are based for example many games with incest, where the incest itself becomes the plot and motivation of the characters.
but were they particularly useful to the story? i don't think so.
that Maya and Derek are siblings has actually damaged both of them, if Maya had hosted MC just for empathy it would have been better than if she had done it to return a favor to her brother, and Derek's role as the controller of his sister's infidelities has certainly not earned him any points. that they are both Patrick's children has so far had minimal consequences and negligible developments
the same goes for Tommy and Josy, a situation that has generated little and nothing, in theory the most important consequence would have been that Quinn wanted Josy in the HOTs to annoy Tommy, but it seems to me that it is a topic already exhausted, there are still the cuddles that Heather does to Josy ... unmissable
the fact that Tybalt has his little family there with him has generated a few moments of comedy and, in theory, the sword of damocles of the video of Jade (extra-optional)
that's fine, but it's this idea of microworld that drives and generates all these theories (see a new professor, and she could be a friend of Lynette's youth, Melanie how can she not be Troy's sister? and if Nora was related to someone else who would really be surprised?)
in practice, the game of the bottle (or of the dildo) has an enormous possibility of generating incestuous acts