Plot Boku wa Imōto ni Koi o Suru

yori , iku yuki twin brother , sister, have been extremely close since born. during childhood, yori used tell parents marry iku. while parents brushed off innocent childhood fantasy, yori knew feelings more innocent crush on sister. when fifteen, feelings have grown stronger before. yori has understood has fallen , hopelessly in love iku, makes him feel ashamed. due share bedroom, yori finds harder , harder resist feelings her, begins distance himself , acts coldly her. iku hurt , shaken harsh treatment towards her, tries remain cheerful , make him smile. 1 day during school, yori gets fight after classmate makes fun of iku, , gets injured. tomoka kusunoki, iku s best friend, tends injury , tries comfort him. in midst of this, kisses him , professes love him. when asks him if enough him, sees opportunity surpass feelings iku. proceeds kiss , has sexual intercourse @ hotel. encounter, however, nothing him forget lust iku. tomoka mistakenly thinks yori reciprocates feelings , calls him boyfriend. yori, however, decides hide iku won t upset. later tries new strategy: go out-of-town high school. days later, yori , iku fight after iku talks dreams of having boyfriend , going on dates. yori gets mad , yells @ iku shut up. iku starts crying. while crying, kisses her, shocks her, , confesses love her. iku taken aback confession , rejects advances because brother. however, sees sadness on face , realizes that, while uncertain if loves him more brother, may lose him rejecting him. kisses him , feels uncomfortable it, yori notices , tells wait until sure of love him. on next several days, try begin intimate relationship, iku growing adjusted yori s advances, , keeping parents in dark. haruka yano, yori s best friend, notices twins closeness doesn t tell them. iku continued torn between guilt of committing twincest , desire remain yori. guilt, however, disappears when discovers yori planning enroll in different high school , has been seeing tomoka. tries have sex yori, in process, rejects because still doesn t love him yet. next day, sees tomoka clinging yori, iku realizes has fallen in love yori. after confessing him, skip school , flee church, remember playing @ when little. try have sex in church, iku tells him stop because feels god watching them. yori agrees , decide go home.

soon after, during school, tomoka talks iku , friends first time yori, angers iku. yori comes in , tells tomoka relationship on before leaving iku. tomoka smiles while crying , doesn t understand why broke suddenly. when mentions yori has else likes, realizes that person iku. iku fights yori on indiscretion tomoka. hugs tries away him, when sees him cry, gives feelings , hugs him back. later day, iku has yori meet , tells him wants make love him right now, shocks pleases him. go hotel, , after iku apprehensive, have sex first time. yori thrilled iku his, feels guilty making cross taboo line , asks god punish him. days later, graduate junior high school , iku realizes yori still plans leave different high school. yori tells yano truth of relationship iku, yano knew asks why still leaving. yori says torn between desire protect relationship , wanting destroy sin of it. yori tries comfort iku, asks why wants go. yori says doesn t want leave, if stays, can t normal couple. however, if city, no 1 knows them, couple. iku reluctantly agrees let him go, promise stay in touch.

yori arrives @ high school, , horror, finds tomoka has enrolled in same school. tries him take back, rejects her, doesn t seem bother her. sixteenth birthday, yori , iku receive cell phones, makes easier them stay in contact. iku visits yori as possible because yori allowed limited visits outside of school. befriends takuma kakinouchi , girlfriend mayu taneda, act confidants. birthday present her, yori gives iku diamond clover ring promise of eternal love. separated yori weeks @ time, iku placed in care of yano, acts protector. yano helps twins see each other @ school , arranges them spend night together. when tells iku knows relationship, assures keep secret. however, spends time her, yano falls in love iku. yori vows never give iku up, , 2 best friends have friendly rivalry on iku, oblivious yano s feelings. twins parents mistake yano iku s boyfriend , take instant liking him, says not couple. when tell iku likes her, iku rejects him because loves yori.

yori realizes tomoka determined revenge against him , iku breaking her. has them caught in yori s room fellow students several times in order expel him (girls forbidden in boys dorms). when yori gets light punishments offenses, takes cell phone picture of 2 kissing , threatens send know if doesn t sleep her. when appears accept deal, deletes picture , tells leave him , iku alone. tomoka, however, refuses listen , continues plan expel him. time later, while yano @ iku s house, father comes home , happily tells wife ran longtime friend, dr. yuugo mori, takes aback. tells yuugo has daughter same age yori , iku, , promised have yori engaged girl. twins mother refuses , asks when said girl similar yori. sounds suspicious yano, tells yori thinks iku might not sister. yori rejects possibility, though later finds out , iku have different birthdays. when iku sneaks school visit yori, tomoka hires group of classmates rape iku. fortunately, yori catches them , punishes them , tomoka. school principal decides expel yori continuing let iku dorm. iku happy of news, annoys tomoka , calls stupid. yori defends iku tomoka s insult, , threatens tell mother twincestuous relationship. doesn t believe her, twins find mother has overheard conversation. again, yano lies, saying tomoka joking , iku girlfriend. mother not believe them, doesn t out of fear, , takes twins home. yori enrolls iku s high school, meet yuugo s daughter azusa. notices yori , iku don t resemble each other while looks yori. makes yori consider possibility iku may not sister after all. azusa tells father of how different twins are, , after invites yuki family on house , sees yori first time, confronts mother. yori witnesses intimate moment between 2 , suspects may have had affair. later finds iku s unfilled birth certificate , photographs of parents yuugo. convinced more before iku may not sister , man believed father not. azusa confesses yori believes half-siblings, , yuugo confesses has been in love yori s mother since college , believes yori s biological father. secretly uses piece of yori s hair dna test, confirms suspicions. tells twins mother of news , shocked when admits knew yori son. had kept secret out of fear of ruining family. yori confides in yano whether or not iku sister, , yano talks family friend, says unfilled birth certificate indicate iku abandoned infant , adopted parents. yori sad , yet happy , iku able live normal couple in public. after this, yori makes feelings iku clear mother, reacts in shock. tells yori stay away iku because siblings. yori doesn t believe , threatens destroy family if tells relationship. mother horrified, takes iku room. making love, mother confronts them , tells them siblings, yori asks answer question iku s birth certificate. tells twins , father twins, half-twins; result of heteropaternal superfecundation. tells them yori son yuugo mori , iku daughter husband. sixteen years ago, when marry husband, yuugo confessed love her, , although had feelings him, refused him. @ moment, kissed , both had sex. tried forget betrayal , slept husband, in same day. 3 months later, found out pregnant twins , unsure father was, until twins born , saw physical difference between them. becomes physically ill after telling story , lashes out @ yori twincest iku.

realizing may separated parents know secret, yori , iku run away home yano s , stay @ beach cottage. yori tells iku if had child together, parents never have separate them. iku hesitant @ idea, agrees, , make love in attempt conceive. later on, go church pray, , yori realizes iku cannot give parents him. realizes have destroyed family , yori blames himself dragging iku commit incest. decide return home , reconcile parents. yori decides leave family , go somewhere can never found in hopes , iku s feelings disappear overtime. before departing, cuts iku s hair (with iku s permission) looked childhood @ request, , slips out when falls asleep. when wakes , finds him gone, iku falls deep depression. however, realizes never did yori , resolves find him no matter how long takes. on next ten years, iku manages university , becomes sales representative yano s company. yano has asked iku marry him, rejects him. during trip england, goes library because thinks place yori go to. pass each other, don t recognize other until after few seconds, , turn around , embrace. in climax, yori states still in love iku.