I only read one a long time ago. The title was something like "My brothers special penis problem". I'm not too sure. The premise was that the black kid was adopted after his parents died. Then a few years later his brother (MC) wakes up at night as his new brother has a problem. The MC goes to get mom and she comes to their room. The MC then spies on his mom and brother fucking. And his black brothers makes a show of it. 

E: Found it. It's in this thread here. https://bullyfucksmymom.com/fapping-materials/help-me-find-a-story/