Can the bank deny my father his money if he has set it up in joint accounts with his children and two of them won't sign to release funds?

My father is residing in a full care home and his rent and expenses exceed his pensions. I am my father's P.O.A. and have been the only one out of three siblings that has ever done anything for him. I haven't had any communication for 6 years when my BIL talked my father into become his P.O.A. BIL had never any interest in my dad's welfare until he asked if he and my sister could take his bank statements home when my father was in the hospital, my father said he could. My father had just previously lent my daughter a small down payment for a town house and didn't realize it would show on these statements as he didn't want "the others" to know. My brother and sister were quite upset about this and shortly afterwards, I found out the BIL went to my dad's place (he was in assisted living at the time) and took him behind my back to a Notary to become my father's P.O.A. Soon afterwards, BIL set up a meeting at the bank after BIL had talked my dad into investing his money into 3 joint mutual funds with our names on each fund with my joint fund with my father being $11,000 less than the others until the loan was paid back, which I was fine with that part of. The loan was fully paid back within 2 years. These funds were also set up in a way where they required the signatures of the fund holders signatures if my dad were to need to use them. My father was 90 at the time these funds were set up and thought he could trust his children to do the right thing. My father really didn't fully understand everything that was said as he has significant hearing loss even with wearing his hearing aids. BIL had invested some of father's money into an account for funeral expenses and any remaining debts at the time of death. He also left my dad $9,000 to last him the rest of his life. My father had realized shortly afterwards that he had made a terrible mistake appointing BIL P.O.A. and asked me to be his P.O.A. and had BIL revoked. I have been my father's P.O.A. for the last 5 years. Now because of father's rent and expenses in a full care home are exceeding his income, not to mention many other large expenses, multiple hearing aids, new dentures, cataract surgery, glasses, private hospital rooms, the list goes on and on. My father now requires the money that was invested in the 3 joint accounts. When taking my father to the bank to transfer some of his money into an accessible account to pay his bills, the bank manager said the siblings would have to come in and sign to release the funds. My father hasn't heard from them in months and ignores their calls and they refuse to sign. The bank manager said these funds can't be released until sister and brother come into the bank and sign over these funds. Well wouldn't you know, they have refused their own father HIS money. Now we have been advised to hire a lawyer. Even with a lawyer coming into the bank with my dad, the bank manager says there is nothing they can do. My dad is living off of the joint account that is in my name which is totally fine with me. It's been a very long, expensive process dealing with the lawyer and this will eventually go to either Discovery or court which will be extremely costly, not for my dad because he will get his money back but for the siblings but they are too ignorant to realize this and very worried about their so called "inheritance." I forgot to mention that my dad is very cognitive and has a letter from his doctor stating that he is able to make financial decisions. My father's lawyer has also stated that she has spoken with him and he knows exactly what's going on. They have accused me of stealing my dad's money, but I have kept every single receipt for the last 5 years to show every purchase which they refuse to look at as it would cost them money to have the receipts photo copied by the lawyer. I believe this is partly the banks fault for not letting my father have his hard earned money. Given my father's age and the circumstances, you would think the bank would have some compassion for the elderly. Has anyone else experienced this problem with joint accounts?