Now that you have dealt with Kleon, return to Myrrine beneath mount Taygetos, west of Sparta. Tell her about everything that has transpired, and that you think your sibling may still live. You will then have to ascend the mountain with Myrrine to confront your past. When you reach the top you will find your sibling at the edge of the cliff. A cut scene will ensue, after which you will have to make some important choices in dialogue options, the main one being 'I don’t want to fight you' if you want to achieve a game ending in which your sibling is saved from the cult of Kosmos and your family is reunited.
If you choose to fight your sibling, they will inevitably die. The fight will play out in much the same way as it did the last time you fought your sibling. The same special abilities will be used against you, only this time you will be better equipped to deal with them if you have advanced significantly in level. The same principles apply as last time; avoid the fire lines which will erupt from the ground, and dodge whenever you sibling charges as you will not be able to block the special attacks. Focus on executing headshots with bow skills and when they are close, use a combination of light and heavy attacks to inflict the most amount of damage.
When the fight is over, you will be transported to your family home and the quest will end.