Horny fisting

I was talking to my stepsister and she briefly let slip that she would like to fist a woman. She wants to feel her arm disappear into a wet cunt. Of course I didn't let her tell me twice and suggested that she could try it out on me. No sooner said than done and so she tries it on me today. Slowly she inserts her fingering into my hole and then continues until her whole hand is inside me. I cum so horny. Oh, God, through my stepsister. Do you find that reprehensible of us?