Hang on, let me recap.
That girl turned her big brother into a young girl, in order to "rehab" him, without being aware that she was the reason he became a NEET in the first place due to his massive inferiority complex over having his role as the reliable brother being usurped, and her endgoal is to marry her big brother to another man, despite the fact that he needs his medicine often to stay a young girl?
That's some extreme fujo stuff even I can't get into. What will happen if her big brother returns to his original, male form after being married?