What does it mean to dream that my brother is lost?

1. Because dreams are contrary to the facts, your brother and aunt may have good luck (brother meets noble people, aunt makes friends), and you can meet them in the near future.

2. Dreams are predictions of reality. If you meet them in the near future, please tell them to be careful.

It's just divination, sometimes not allowed. If you don't believe it, think of it as your brain sorting out memories.