It seems like the sister will eventually kill her brother and then commit suicide, but I predict that the sister will harm Sa-chan and cause the brother to "take responsibility." (I mean, why not? He already blames himself for being drugged and raped by his sister, so would him feeling guilty if his sister were to harm Sa-chan be so farfetched?).
Frankly, I would prefer a murder-suicide scenario to the more clichéd ending with the two siblings going incognito by moving to the countryside; that ending is only romantic when the love is mutual, not when the sister is a psychopath and the brother is in a drug-induced stupor.
Also, am I the only one who wishes that MangaDex had a love triangle tag, which in this instance would seem far more appropriate than the harem tag?