I tried adding my biological brother to my family tree (living). However, when I tried to search for him using his ID number, I encountered an error "ID number not found". Also, he also has an account on family search.
Profiles of living persons can only be seen in the private space of the person who added that profile. In other words, only you can see that profile with that PID.
If he has account on FamilySearch, and has created a profile for himself, only he can see it.
So further, what you need to do to add your Living brother is to create your own Living profile of him.
Understanding the "Private Space"