Alright, so this is how you get the best ending for Sosiels quest line, where both of the brothers live.
1. At start of chapter 2, do Sosiels quest in the graveyard, you can pick any option you want regarding the cultist.
2. In lost chapel area, once you get Sosiel back, he will have a special shield called ''Immaculate Petal'' equiped in 4th weapon slot, don't sell that shield, its needed for the good endings.
3. After finishing lost chapel, talk to Sosiel in warcamp about what HK told him, you will be presented with 3 options, pick this option ''You don't know what your brother has endured in this war. Perhaps the person you once knew is gone''.After that, ask him what Irabeth told him and you will be presented with another 3 options, pick this one ''The battlefield changes people. Your brother isn't the first to be hardened by war''.
4. In chapter 3, wait until the impostor arrives, talk to him in jail and get the camp location. Go there, fight the purple worm and come back to Drezen. Exhaust the dialogue with impostor and pick this option at the end ''Wait. This is only one person's side of the story, and a liar's at that. Let's find the other two survivors and ask them.'' Then, go to the lost chapel and talk to the 2 deserters. Exhaust their dialogue and at the end you will have 3 options. Pick this one ''Your brother hoped to keep his heart pure - but he couldn't. Alas, this happens to many people in war.''
5. In chapter 4, bring Sosiel in flesmarket in middle city and talk to the black mask guy, he will tell you that Trever is locked in the arena. After completing the arena questline, bring Sosiel and talk to the arena manager to arrange a fight between Sosiel and Trever. Pick any option expect attacking him then pick  ''Sosiel raises his shield'', if you get the option ''Trever... What have they done to you? No, that's not what matters. You're safe now, its over. I've come for you. Let's go home'' you got the best ending.