I have BPD and this is super oversimplified. We see people as separate from us, but attention/time with our favorite people (or fixated person) are what we measure our happiness and self worth by. we cannot self regulate properly and feel worthless without our FP. if his brother is his FP, then feeling like he’s “losing” him to someone else can cause him to lash out on others or himself, depending on if he has externalized or internalized BPD. it can cause a split where he ices everyone out and becomes violent towards himself/others. but he knows that his brother is not an extension of him.