It's not cheating if it's with your stepsister

I have a girlfriend, her name is Shiori Hoshino. She is known as the “Ice Emperor” because of her overwhelming appearance and cold personality. And surprisingly, she is an anime fan, and also considers herself an otaku. But in reality she hides a secret, and that is that she has a phobia of men. Although for some strange reason, she usually feels at ease with me.But, there is something that I have not been completely honest with her about. And that is that I maintain a secret relationship with my stepsister. Although, it’s not infidelity if it’s with her, right?