Complex sentence: The Social Democrats gained three seats; however their leader, Ingibjörg Sólrún Gísladóttir, was not elected, as she had only placed herself 5th on the list of candidates for her district.
Split into simpler sentences: The Social Democrats gained three seats however their leader, Ingibjörg Sólrún Gísladóttir, was not elected. This was because she had only placed herself 5th on the list of candidates for her district.