She hadn't. Not directly, anyway. Her mind was on his hands and arms and chest and butt (which she grabbed once on accident and once on purpose during the scuffle). She hadn't registered that part of his abdomen was suspiciously hard. So, when he lifted off her, she immediately got up, spun around, and tackled him. He was mostly thinking about keeping his rock hard cock away from his sister rather than fighting back, so she easily pinned him to the ground in turn. Not that it did him much good. He ended up on his back, his sister straddling his waist and pinning his arms up behind his head....