Translation:My father has eight siblings.
Kyou きょう is a loan word from chinese, where it is pronounced as xiong in mandarin and hing in cantonese. It literally means “elder brother”. だい written as 弟 in kanji literally means younger brother. So 兄弟 actually means “elder brother younger brother” and is used in chinese as collective word to denote brotherhood. It is used not only with respect to blood relations but also to other brotherhoods by association where seniority is hierarchic such as in the criminal Triads gangs, in martials arts associations or even when men swore oaths to treat one another as brothers although this is a bit ancient and rarely seen these days.
I answered this question correctly but used the (correct) kanji 兄弟 rather than the kana きょうだい and my answer was marked incorrect. 兄弟 has been accepted in place of きょうだい on other questions in this same lesson, so this was strange to me.
My full answer was 父は兄弟が 八人います。and the "correct" answer it gave me to use instead was 父はきょうだいが八人います。
Just as a heads up!