The sister came running from school because she had to pee and her brother tripped her and held her down until she pissed herself. They had sex and continued to have sex until they grew older, with the sister being more interested in having sex and the brother losing interest. The manga ended with the sister cleaning brother's ears. That's all I can remember.

pretty sure thats the first story in Comic Ped by Hatch

stupid photobucket, wanting $400/yr just to display your images