Complex sentence: Sievernich has possibly fathered a child with Frida Schelling (whom Johan had killed for him); if so, Sievernich is the first (and, so far, only) "monster" to have a living child.
Split into simpler sentences: Sievernich has possibly fathered a child with Frida Schelling (whom Johan had killed for him). If so, Sievernich is the first (and so far only) "monster" to have a living child.