Ben, there was a first time you had this response to your big brother. During that incident, you decided something about yourself and ever since then all of your interactions with him have been run by that decision. That is the fixed mindset.
So I wasn't asking you to remember a significant incident, but rather, the earliest time you can remember feeling this feeling of -- what is it -- freezing up, shutting down -- with your brother. Something happened and it might not strike you as a significant memory, except that you'll be able to "feel" the memory.
You up for that?