Is cash in a safety deposit box at bank subject to Medicaid requirements for eligibility? In other words can it be hidden?

My mom and my brother who is POA and named joint owner on all her bank accounts, CD’s etc. have been socking cash away in a safety deposit box at bank. I get the sense they think this money does not have to be reported. Brother is POA and refuses to share information with me, insists that he will be the one to take care of her and refuses to seek legal advice about Medicaid and how to preserve her estate. On one hand I don’t want any part of her financial issues since he is so hostile to me. On the other hand I don’t want to see any potential need for Medicaid jeaopardized if/when there is a look-back.