Should i tell her the truth ?

My sister fiance was d one dat secure d job am doing right now for me, he also work there as well , we stay in d company's quarters but not in d same room, there was a day he came begging me dat is female friend will be coming, dat i should not let my sister know about it and i promise neva to tell my sister and dat day d girl actually came d way he was hugging, kissing her shows is beyond a mere friend and d girl slept in his room, ever since then he has been bringing different girls. M y sister is in school she only visit us once in a while, should i let my sister know what her fiance is actually doing or i should over look everything and concentrate in my job.