I was confused for awhile 'cause the title is "I fell in love with my girlfriend's brother" and not "I fell in love with my sister's boyfriend" but I realised the title was from the seme's point of view,,,
I was also confused at first....
I just realised that now that you said that
I was confused for awhile 'cause the title is "I fell in love with my girlfriend's brother" and not "I fell in love with my sister's boyfriend" but I realised the title was from the seme's point of view,,,