Complex sentence: Almost all support of Nobunaga that would have been had from Nobuhide's retainers went to younger brother Oda Nobuyuki, leaving Nobunaga with Hirate Masahide and his father-in-law Saito Dosan, whom he had never met before.
Split into simpler sentences: Almost all support that Nobuhide's retainers would have given Nobunaga went to younger brother Oda Nobuyuki, instead. For support, Nobunaga was left with Hirate Masahide and his father-in-law Saitō Dōsan, whom he had never met before.