I helped my normally defenseless sister-in-law stretch, but her beautiful protruding ass got me fully erect! When I finally lost my patience and inserted my penis, I instantly ejaculated because it felt so good. Seeing my erect penis that still didn't subside, my sister-in-law got horny and straddled my penis! She even straddled me in a low position like a predatory beast and pistoned me hard! I was so stimulated that I almost ejaculated instantly. I couldn't let it end like this, so I desperately rebelled!