ESH.

He definitely screwed up, BIG time. He has breached attorney-client privilege. There is absolutely no justification for looking at your file, and even less reasons to share it with others. He is subject to very serious professional consequences. If this breach is proven, not only will he not be a lawyer, he won't be able to work as a paralegal or even a secretary in any law firm in the past.

With that said, do you really want to ruin his career and possibly his life too, just because he knows your finances? He was definitely wrong, but as you mention, the consequences are extreme. Even though you have every right to be angry, your family are going to think you overreacted if you ruin his life because he was curious about your finances.

Getting him dragged into a professional misconduct investigation isn't going to change the past, and (hopefully) this was an unusual case because you are family, and he's not broadcasting other client secrets. Consider carefully whether it's worth ruining his career and ostracizing yourself from your family.