Imouto is becoming real dark, real fast. The look of utter disgust she gave onii-chan after he confessed that no one else compared to his gf in the last chapter was pretty incredible. That expression which stated "You f*cking idiot brother! I see now that I need to become serious in this war of affection...I won't lose to that wh*re!". And now we see in this chapter that little sis is already working to sabotage bro's relationship with the girl.
I don't see this story turning out as anything less than a total bloodbath, but I'm anticipating the creativity and mayhem that the Poor Little Sister utilizes.