Jurij's brother, Erich, participated in the coup d'état that happened around 10 (or so) years ago. Mostly described in chapter 8 and chapter 28 if I remember right. Liebknecht is the one who orchestrates it. Baltzar pretends to help them but leaks intel to Weißen's military. Shit happens, Jurij's brother apparently commits 'suicide' (along with all other insurgents), Lieb escapes. Jurij blames Baltzar for his brother's death, saying he used Erich, and wants revenge.
Turns out Erich was working with Baltzar to undermine Lieb's coup. It's now unclear how exactly Erich died or who killed him. The insinuation is that it is Liebknecht (but read chapter 28 as it might be something different).