Complex sentence: Heimdall then uses near near-omniscience power to locate the scattered souls of the other Asgardians with the exception of his sister Sif, due to Loki's own spirit secretly utilizing spells to shield Sif from both his and Thor's detection.
Split into simpler sentences: Heimdall then uses near near-omniscience power to locate the scattered souls of the other Asgardians. However, Heimdall is unable to locate his sister Sif, due to Loki's own spirit secretly utilizing spells to shield Sif from both his and Thor's detection.