Can you sue a facility if they blockaded you from a parent?

Now that you are guardian I would put this behind you. Your brother had POA at the time. The facility was doing what they were told. You did what u needed to do. So go from here.

Gerirocco, pro bono attorneys in my experience and to the best of my knowledge offer their services to people at financial levels which wouldn't support hiring their own attorney.   Pro bono focuses on financially challenged people.   

Your suit would very likely not fall into this category.   

Another issue:  is your proposed cause of action primarily against (a)  your brother or the facilities, and/or both, and/or (b) b/c you were prevented from seeing your mother, or (c) for financial abuse and/or fraud?

I can't imagine any pro bono attorney taking on a case if based on (a) or (b),  and it would probably be unlikely that one would consider the challenges of (c).    That would require an attorney with support staff and finances to pay for the documentation that would have to be ordered.

When you achieved guardianship, did you raise this issue with any legal professionals?  

If you're serious, search the local bar associations for attorneys who handle elder abuse (a civil case) and financial fraud (also a civil case if YOU sue but a criminal case if you involve law enforcement, which I think would be necessary to get the data to prove fraud).

I'm not an attorney but I cannot imagine any viable court case against a LTC facility following the orders of the legal POA at the time. The fact the POA was later overridden by a guardianship doesn't change the facility's obligations at the time.