You failed in your attempt to keep a long story short. This is 16 years old so some of your causes of action will run up against the statute of limitations (SOL). The IRS will not care that he received money you should have, unless he failed to declare income or pay taxes. Many of the records you need, such as signing at a bank, might not be available. At the end you write "signed the house over to me". Maybe that is your 1/2?
You want to do leg work. Okay, start with what crime or tort you think he committed in what state and then check the SOL. I think you will find that you are SOL (with a different meaning).
You didn't say whether a probate was commenced. That would be a starting point. Generally, the petitioner would have to file an inventory with the court that would include a list of all the assets. If you petition the court and are appointed, you would then have the authority to contact companies to gather information about the assets but the fact that it is so old, the companies probably don't have the records anymore.
If the investments were POD (payable on death) accounts or named your brother as a beneficiary, then they are not probate assets and belong to your brother unless you could prove fraud, something that would be almost impossible to do 17 years later, but not impossible.
Unless you can provide sufficient proof of fraud, I doubt that any attorney would take this on a contingency basis.