cute little story.

Girl is the gender bent older brother, caught the dreaded TS disease.
There is a cure/treatment, however it is very expensive. So he gets the younger brother to help with sex as that can also keep it from killing MC.
Seems parents are aware and are ok with it. The story is basically the MC being reluctant, but as they have more sex becomes more submissive. By the end of the second chapter they try resisting but realize how weak they are both physically and mentally. They just give in and will let the younger brother just do what he wants.

Base +8, yuriftw +7

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Base +6, ineman -10