I an 20 yrs old girl . i had a boyfriend who cheated on me . he said hs mothr hd fixd hs marriage 2 hs bro's ?

Wife's sis n he cant go against dem . he knew dat hs mothr wntd dat bt evrything was in his hand bt one day his bro's mother in law had an accident she was in serious condition n she holded his hand n askd him 2 marry her daughter . nw i dont understand when he knew he wont b going against his family den y ws he in a relation with me ?. what was my fault ? wat shud i do now how do i forget him