What does Song Meiling mean by calling Chiang Kai-shek his brother-in-law

Not a brother-in-law Song Meiling studied in the United States as a child and was deeply influenced by western culture. Because of this influence, Song Meiling lived a western-style life all his life. Like westerners, he called Chiang Kai-shek a "favorite". Song Meiling (1897 from March 2003 to/10 2003124 October), a female from Wenchang, Hainan, was an influential celebrity in the modern history of China. He devoted his life to the people of China, War of Resistance against Japanese Aggression, opposed ethnic division, and looked forward to the peaceful reunification of the two sides of the Taiwan Strait and the prosperity of the Chinese nation.