was+p.p는 단순 과거 시제로 (언제부터 였든지) 과거에 그랬었다는 거고 현재완료 have been+p.p는 과거부터 현재까지(과거의 어느 한 시험부터 지금 까지)를 의미해요. I know I've been a terrible brother: 난 내가 (지금까지) 나쁜 brother이었다는걸 알아 I know I was a terrible brother: 난 내가 나쁜 brother이었다는걸 알아 ☺

For the first sentence, we know that the speaker was a terrible brother and is the same now as well. For the second sentence, we do not know if the speaker is still a terrible brother. He may have changed, but we do not know for sure. The speaker was a terrible brother, and it only remains in the past.