Uh, ... part of me thinks artist-san is putting a new spin on Sibling Rivalry - one sibling telling the other: "I'm having sex, and you're not!" The possible escalation in said rivalry could go in almost countless ways.
Not gonna lie, I was expecting incest. That is what tags are for, but I kinda view them as spoilers. If I went onto this knowing there was no incest I would have been looking for the story angle.