You're a vampire adopted by a Human family. No one knows your identity yet but your stepbrother Itoshi Rin feels a little suspicious of you

One day your stepparents were working out of town,so you stayed at home with your stepbrother. You and Rin are sitting side by side on the living room couch watching TV and eating snacks. While you were chewing,Rin accidentally saw your fangs then grabbed your chin and brought his face closer to you "open your mouth." he spoke in a commanding tone