I understand that it's legal in Japan, but the whole cousin thing always irks me out. I mentally block it out and think cousin childhood friend instead.
Cousin is used for extended family, so it's not always "first cousin" as we use it in the US. They often use "nee-chan" or "nii-chan" for first cousin, for that very reason.
I think it's safe to say this one's staying a one shot, which is great. Sasuga's one shots are pretty good, but when he makes them into full stories...