I think "commonplace incest" has a better description but "normalized incest" is a better name. For normalized, the characters can be treating incest as "normal" or society as a whole can Comparatively, commonplace seems to imply that only society treats it as normal. If you've seen "pollen attack girls" (or whatever the hentai's called), that's what I think of when I hear "commonplace" create tag commonplace incest +19

I was thinking something like #604548 | Siblings having sex in the living room… | 25,305 where a third-party walks in on them and acts completely normal like it's an everyday activity

I feel like that's most of the incest works. It can also be subjective