I wouldn't tag this as netorare given the brother got dumped before he fucked his sister, and his sister gotten dumped without her boyfriend knowing she's fucking her brother. So how can this be netorare? It's not like the girlfriend or boyfriend of the brother and sister knew anything.

This isn't ntr. the point is a humorous setup for this doujin logic "wholesome" couple. we don't experience the perspective of the people getting cheated on.