Complex sentence: The brand was retired at the start of the summer flight schedule, on 31 March 2013, and was replaced by the new brand "Austrian myHoliday" which is no longer an airline but only used to sell Austrian Airlines' own leisure offers.
Split into simpler sentences: The brand was retired at the start of the summer flight schedule on 31 March 2013, and was replaced by "Austrian myHoliday". It is no longer an airline but branding that is used to sell Austrian Airlines' own leisure offers.