Step dad will not pay for mom's assisted living center because his Attorney said so... Can I do anything?

It sounds like your stepfather may have been advised to do a "spousal refusal" or "spousal impoverishment" with regards to your mother and Medicaid. I don't know what state you are in, but it seems to be an option being used by some folks with money that is primarily related to one wage earner or a person who got a large inheritance or came into marriage with larger number of assets. Most folks don't realize that division of assets doesn't come into play if they stay married and one of the spouses gets a guardian so a lot of bank accounts etc. can be moved or assets retitled before Medicaid is satisfied. Big problem when Medicaid reviews financials and probably disqualifies person right away. Does your mother get Social Security? Is there any of that money in the accounts? Then he cannot withhold those funds from her conservator. This sounds like a hot mess that will require the intervention of state agencies if his lawyer will not respond. Is your lawyer an expert in Medicaid and elder law or an expert in guardianship? If mother is receiving any pension or Social security, you need to set up accounts for deposit of those funds with it changed from any accounts that stepfather is on. You may need to get the judge that granted conservatorship involved to issue subpoena for records financial so that you can sort it out with the nursing home. You may need to get Adult Protective Services involved if his refusal to cooperate is causing your mother to not receive necessary care. I am sorry that your poor mother and you are going through all this while facing this care diagnosis.

So, as conservator you have the ability to access their joint accounts. I would ask your lawyer before moving any money, but I don't see any reason you can not transfer her funds into a separate account and pay for her care from it. (The reason I would move the money first is to prevent him from doing the same thing and blocking your access).
Beyond that, there is an obligation for step dad to care for his wife, he can't just walk away from his responsibilities, depending on where you live the money is considered community property regardless of who contributed the most. I can see that this will become a complicated legal tangle if step dad won't cooperate. Also, be certain your lawyer understands the ins and outs of Medicaid, which is a specialty.