I am very sorry for the delay, but I was finishiing another answer, and it took a bit longer than I expected.
You've got a lot of things going on here -- everything from theft to a conspiracy
to commit murder and the neglect issues involving your brother. If you are interested in having the case prosecuted, you would have to report the offenses to the police and show them the evidence that you have. You need the police where the crime took place, so that would be the state where the accounts were and where your brother was.
If they agree with you that this matter should be prosecuted, they will turn their file over to the prosecutor who will file criminal
charges. Once that happens, this is not your case any more. It belongs to the state of Wisconsin and the prosecutor gets to call it as he sees it. That is, he, and not you, get to decide what crimes your evidence makes out. And he and not you get to decide how aggressively he prosecutes.
You are not going to get to help your family in this. It will be expected you help the prosecutor. If you're not looking to go that far, then you would be better off not reporting it. If the evidence is as serious as it sounds like it may be -- not the theft, which is only money -- but the deliberate planning to take a life (assuming they put this into operation) -- they are likely looking a years of prison.