Not stuttering in front of my little brother?

Not stuttering in front of my little brother? So I’m a 15 year old stutterer, and I have tried many types of therapy. I recently found that when I am either alone, whether it be speaking to myself or reading, I’m totally fine. And I also found that when I speak to my 8 year old brother, I also don’t stutter, I suppose it’s because he never brought up the stuttering topic that I feel relieved when i speak to him? And whenever I speak to anyone in a low tone, I speak fine too. Why is that?