Man, this is a pretty bad plot twist.
Both faction searching for the main duo was unable to find them so the author makes a non sensical and unwanted magical flare pinpoint their location to both sides.
The incest gimmick is not bad, a isolated tribe with a unique tradition, the MC with our world common sense and that clearly wasn't much on internet so he decided to run away from it, a yandere sister that doesn't want to let him go and have the whole village permission, they click well together, and if the author decided to not put them together it remains a gimmick.