They are, yes.
It’s technically incest generation by generation for hundreds of years. I have to wonder when it became the norm. Instead of marrying outside which probably only happened the once when the two families were probably first paired all those hundred years back. Seeing as it’s only a gene pool of two families for hundreds of years it’s worse then cousins they share the exact genes so it’s more like siblings because of the lack of variety.
not to mention they said his family has provided females for them for generations(●￣(ｴ)￣●)
Yes because nagi's dad is rio's mom brother so they are sister in laws and nagi and rio are cuisines in some weird way that can happen
I can't remember if they mation this before but in ch.5 page 27 Nagi's mom calls Rio's mom sister... so Nagi and Rio are cousins?