First, Reiji and the sister tried to chemically neuter the elf brother.
Reiji explain poorly to the sister that he want to give her the drug to chemically reduce his lust. He said "First, you should try to gradually reduce his lust." Implying that there are more steps he was going to do later, at least to the sister.
This was what the British government did to Alan Turing, the man who was one of person who had major role in defeating the Nazis, laid foundation to the current computers, among many of his accomplishment. It was immoral then, it is still immoral now.
The elf brother character was not much different from the crude male character who behave that way toward women. Instead of rough looking ruffians, he was a bishonen elf.
The sister speech bubble in the last panel here:
She specifically said "cure nii-san's man love". She certainly wasn't talking about his libido. Regardless of what Reiji's intention, that's what the sister said. The sister who seems to not understand little else other than wanting her brother in a "Flowers in the Attic" way.
For me, I felt that the author was ignorant of the subject matter. This whole scenario use the typical perception of how straight people see gay men as predatory toward them to make a character that people closed to him wanted to *fix*. This is all played for laugh, with little care to the subject matter and the implication.