It sounds like you made a wise move with your job. It's not an uncommon maneuver, and your current job might raise your pay to keep you. I would suggest other means of support in addition to your fiance. This might help you feel more balanced.
As for your fiance's brother, it's unfortunate but it's not your responsibility to shoulder everyone's feelings. If he hasn't been sentenced yet, it might not be as bad as what you are picturing. Focus on the now, not the "what ifs"
Take a deep breath, and try to shut down that anxiety spiral. Remember that so much of what we worry about never will happen.