Boyfriend's sibling(s):

Donghyun has a younger sister, name unrevealed
Hyunseong has an older brother, name unrevealed
Jeongmin has an older brother and sister, both names are not revealed
Youngmin and Kwangmin, besides themselves, have have a younger brother named Hyunmin
Minwoo has a sister named Eunbin, the main image is her.

Source: [xx]