Song Mino was a middle child. 

Jiyong, Jennie and Mino all have different mothers. Jiyong is the only legitimate child of the Kwon patriarch, who died 3 years ago. But before his demise, he admitted to his wife about the other kids he had with other women.

Instead of hating them, the Kwon matriarch searched for them, to see their condition and give Jiyong the siblings he never had.