Now it gets a little more interesting because the story doesn't directly tell us that Tanuki loves his sister in a romantic way, maybe it's just familial love.
It would be interesting to give a story of step-siblings who just have a real sibling relationship rather than an incest story.
And I don't think it can be called NTR since it seems like Tanuki invited his friend. I think Tanuki's plan now is to use his best friend to protect his sister while he's not around.
And when I went back and read the first chapter again, I realized that Tanuki's friend was the one who unknowingly gave him the idea of "making his sister understand". Now he has to take responsibility for his words and become part of Tanuki's plan, lol.