IENFH-009 - Caught by my sister while sneaking a peek, now she won't let me finish until I fill her up

Helping my sister with her thesis, I couldn't resist her defenseless sleeping form. I tried to sneak a touch, but she caught me and took control. She locked me in a scissor hold so I couldn't escape, and now I'm forced to dump everything inside her. It's wrong, but I can't stop!