My guess from what I’ve read is that the older brother was not in the young brother’s life for quite awhile. The younger one was normal and good in school and had a future, etc. After high school graduation he went to college. None of his high school friends knew that he had a brother. Apparently sometime in the past year or year and a half the brother comes back into his life and radicalizes him. Won’t be surprised because of the age gap that he might have idolized him and yadda yadda yadda.
Again though it is what is called radicalized/militant muslims who are the middle of the activity.
Again also be careful who is in your circle of influence…