There are several issues. In order to protect his interests, your brother and the seller should not only execute a detailed asset purchase agreement that fully outlines the terms and what's included in the sale, but he should conduct full due diligence to determine whether this deal can/should be concluded. He really should speak with a qualified business attorney in his area before proceeding.
You might want to clarify what you mean when you say that your current lawyer is in breach of your contingency agreement? Did he change his mind and wants to do it on an hourly? You should also be advised that even if you do secure new counsel, your existing counsel will probably have an attorney's lien on any proceeds you do recover to make him whole for the time spent on your case.
It honestly depends on your financial situation, and whether you qualify for a Chapter 7 or 13 filing. While both debts are theoritically dischargeable, your financial circumstances may mean that you have to do a repayment plan as opposed to a straight discharge. You should have a consultation with a local attorney who does bankruptcy attorney to determine what your circumstances would allow.
Your first step is to initiate a summary eviction proceeding against the tenant. That's the only legal way to remove the tenant from the premises.
As for collecting what's owed to you, that's an entirely different story. As the addage goes, you can't get blood from a stone. Assuming you get a judgment (keep in mind the tenant could have valid affirmative defenses which could result in an abatement of the rent you claim due), consult with competent counsel about your options. I assume the...
While it's unlikely that you have any recourse against any of the vendors (their contracts are usually pretty solid about protecting non-refundable security deposits), you may have grounds for trying to go after your ex. Additional details would be very helpful. Was he for example on any of the contracts? The merits of your case will be entirely dependent on the specifics of your case, so I would consult with local counsel.
Attorney Fromm is correct. Your situation will be controlled by your particular fact pattern, and, in particular, the order of who passed away. It's be to consult with local counsel to confirm your rights. A qualified estate/probate attorney will be able to quickly assess your situation.