Seconded. I'm personally not that big a fan of the theory that MC and Nami are the spawn of William or Leia because that's way too predictable. It's also rather lazy, to a certain degree. In my opinion, if it were truly that simple, the quality of this story would drop significantlyღ
Thirded
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
. In my opinion, MC and Nami might be Willi´s offspring insofar that he could be a sperm donor for Daphne (that was a possibility mentioned in WIAB) since Nami lokks very, very much like Daphne´s daughter. For me that is the only theory which might be true about their creation with Willi in it.
The Zane family might be dysfunctional and rife with infighting, but they still look after each other. Were MC and Nami really the offspring of Willi and/or Leia, they would not have been "parked" with Nojiko. For me the whole setup of their family looks like they were close to the Zane´s, but not this close.
I theorise far more that the cousins mentioned by Ayua might contain one or two whose parents are Willi and Leia, maybe even Kati.