The short answer to the LW's question is: "Nothing you can do will fix the relationship, also you would benefit from therapy."

The closest thing to usable advice Annie gives the LW is that the relationship between the brothers won't change unless they both want it to, and they do the work.

It sounds as though both LW and the older brother have been engaging in wishful thinking. The previous history described (serious accusations, followed by jail time for one brother and foster care for the other) and the recent threats do not suggest that a surprise visit would be a good idea.