Complex sentence: Sieg never told Arcia why he pursued Delacroix even though he did say the two know each other, and because of that, she briefly lost her faith in him, and set off to find Delacroix alone.
Split into simpler sentences: Sieg never told Arcia why he pursued Delacroix even though he did say the two knew each other. Because of this lack of trust, Arcia briefly lost her faith in Sieg and set off to find Delacroix alone.