They don't... they just learn how to live with the memory of it
Time and separation from this person seems to be the only real "medicine". You have to be sure to not hold on to any hate or shame you may experience from it as well. Talking to a few very close and trustworthy people helped me.
Try to remove yourself as far away from that person as you can and excommunicate them from your life. I do not think you can get a lasting relationship once that trust barrier has been breached, there is no going back. They endure lots of therapy and CBT. It is a sad state of affairs when family members do do this.
I just realized that it happened, it was wrong, and I will not let it ever happen again. I survived and I am still here and ok. So it is now water under the bridge and I can move on and be happy.