A stepbrother is read as a bond formed by circumstance rather than birth. Dream books link him to sharing space, attention and identity with someone you did not choose.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.