She was on her back with knees up at her chest and I was kneeling at her butthole with my lubricated dick in hand in a split second. I looked down and got a greedy look at her sopping wet pussy, but that was strictly off-limits. After a bit more hit and miss, I found the correct angle of attack. I pushed the tip in. When my cockhead entered her anus, her eyes opened wide. This was it! She looked up at me and said 'Do it, baby brother! Do it! Fuck my asshole! Buttfuck your big sister!' I pushed it in further, then pulled out a bit; in even...