I am sorry to hear about your brother. May Allah grant him Jannat and grant you sabr.
Allah (SWT) has created a system in which causes cause effects. This world is a place of trial and also it is temporary and has to be left behind one day. Consequently everyone goes through tests and everyone dies. The worth of each person in God's eyes is not about how long he lives in dunya but what he manages to do during his time in the world. If you ponder over this, Inshallah your doubt will be dispelled.