Life began with self replicating organisms, whose replications would produce new organisms with slightly different DNA from themselves due to mutations, but the majority of the DNA of the new organism produced would be inherited from their parent(s).
Under certain conditions the probability of replication would improve for these organisms, while under other circumstances these chances would decrease.
Organisms who were more likely to act in ways that would produce conditions that made then more likely to replicate would replicate more often.
Thus the organisms would develop an internal system of reward and punishment to incentivize them to seek out the circumstances most favorable to their potential at replication.
Hence, we have reward emotions and sensations like happiness and pleasure, as well as those for punishment like pain and misery. We also have the state of desire that causes us to seek out the reward states, and the state of fear that causes us to avoid the punishment states.
This extended not only to the replication of our own DNA, but also to the replication of our kin, with whom our DNA is shared due to our most probable degree of relation to them.
There is no morality, only emotional and sensations, only rewards, punishments, the desire for the former, and the fear of the latter.
The student pursues his own happiness above all, and by the same principle he avoids his own misery, he loves his sister, and so whatever she feels causes him to feel the same way, thus her happiness or misery is his own.
When his sister is raped, she feels sadness, this makes him sad as well, and he desires to cause suffering to the rapist because it would make him happy if he were to do so, the rapist would fear an encohter with the student as a result of the threat he poses.
No morality involved, it's just people pursuing their own happiness and avoiding their own misery. All in alignment with the principles of evolution. Each character of this story acts in ways most conducive to the replication of genes within the society in which they live. And their feelings and sensations drive them to their actions.
When we look at the net effect of people each acting on their own personal preferences, we see the emergence of a common culture among these people that represents the generalized consensus of all the individual preferences found within that collective.
As certain acts are responded to with a desire to kill or otherwise bring harm upon the perpetrators these actions, the types of individuals who wouod perpetrate these kinds of acts are either wiped out or eliminated become a minority, and the acts themselves become pretty universally accepted as being wrong among the surviving members of that society.
FYI we are currently seeing 2 wars being fought in the news whete people are killing other people over a disagreement wirh each other's preferences being in confluct with their own.
There doesn't seem to be anything here yet