Isn`t Justin the only child?? or He has brothers and sisters????

no.
justin and christian are not brothers. christian is justins ex girlfriends little brother and their really close. but he does have a little sister.

i think hes got a younger brother

Justin and Christain are best buddies, and Justin once went out with his sister, Caitlin Beadles, but they're not brothers. He does have a younger sister and brother,Jazmyn, 2 years old, and Jaxon, 7 months old, both half-siblings froms his dad's marriage. There are rumours that he also has another older sister and a 12-year-old brother from other family connections, but i'm not sure. Hope this helped.