I'm guessing from your question that your brother has joined, and that he created his own profile. To connect with family who are already here, he will need to open the EDIT screen for his profile. Under Edit Family, in the Add Father field he can enter the ID for the existing profile for your father. Same thing with Add Mother.
If you already created a profile for him, and he created his own separate profile, the profiles will need to be merged. I've never dealt with merging two profiles when the newer one is an active member, so I'll leave the question of "how to do that" to someone else.