Not like bros and sisters doing it, but like a clan in ancient japan would have done. Usually it would be a group of people that would have children then their children would marry each others children and so on and so forth. That's the only way you can explain how they were all able to unlock the sharingan. (you do notice almost all Uchiha have basic sharingan right it's not like some super epic secret amongst the clan).
Okay with that being said, in modern medicine and science. Scientist have done test to see how little people it would take to reproduce and repopulate an area. They found it take's as little as 10 couple's (10 male 10 females all healthy no family medical history) to successfully interbreed and produce healthy children that can then have children that will breed so on and so forth. Multiplying throughout generations eventually ending up with say a clan. The only thing that keeps this successful is if they be sure to breed with someone that is far enough away from them in the gene line (what you would consider a 3rd cousin). Isn't our DNA amazing.
So now taking that into account they all could have been distant relatives that just kept marrying into the clan. Thus producing a specific piece of DNA unique to them through the many generations.
I hope that was clear enough.
Edit: I checked the article it was 16 couple my bad so 36 people.
Hell yeah. I'd be like "You know lady Tsunade the reason why I wear this mask is because I have been blushing since I first seen you."
But then again if anything goes wrong the book doesn't have a epic right hook.
