I think the person who can best judge whether the incident was significant is you, rainbow. Did you get a chance to explore it in therapy? Or did you tell her you wished that she found it significant? That itself seems like it might be important.

From what you've written, that seems like it's probably true. So instead of one incident, maybe it's 1000 little incidents, and maybe remembering some of them can provide clues. Maybe this incident with your brother being late to walk you home was one of those thousand and you can learn something from it that will lead to other clues and to more knowledge and insight of what you experienced in your past. Sometimes in therapy I have followed just a flicker of feeling or memory and it has shown me a lot.