It was supposed to be just intercrural sex, but it developed into a full-on insertion! My sister, worried about me because I was a shut-in and about to drop out of school, kept coming to my room, and I couldn't help but get annoyed, so I half-jokingly said, "I'll do what you say if you let me do it," and she agreed to intercrural sex! But when my sister's pussy became slippery after being rubbed against her crotch, she accepted my dick, and it developed into incest!