Mom passed. I found out sibling spent money out of her account for his own bills. Bank tells me I can't do anything about it. Why?

I am assuming brother did this prior to your mother’s passing? If so unfortunately there is not much the bank or you can do.
Did you become administrator because mother died intestate (meaning left no will)? Did your brother provide care for her? My brother passed in June with no will and I had to apply to be the administrator of his “estate” which was nothing but a checking account.
How much was taken? Was her funeral pre paid? Have you spoken with your brother about it?
My guess is nothing can be done. The injured party (if indeed she was) is gone and can’t be summoned for information.

How much money was it?

If Your Brothers Name was not on Your Mom's Bank Account then the Bank should not have given Him money from Moms Account, hence the Bank is liable here. It is obvious why the Bank will not allow You to file for a fraud claim.
I would advise You check this matter out with some Banking Authority, but rest assured Your Brother will one day answer to God for His actions.

Let me tell you. The banks don’t care about stuff like this l, they would rather protect the criminal than do the right thing. You can file a police report and they will probably look in to the matter but since your mother is not alive to tell them she didn’t give your brother permission to take the money, the DA WILL NOT file charges and prosecute. Been there, done that with both a bank and the DAs office. My MIL’s boyfriend took $20,000 out of her personal checking account an hour after she died. When we went to the bank a week later to try to consolidate her accounts, they told us her boyfriend had just been in to take care of their joint account and they knew she had died and had been on hospice in bad shape because he told them. The guy never stops talking and they thought he was just wonderful, they said he is “a real honey doer”. HA! Anyway we gave them the death certificate and they KNEW she died June 1 and they went over her personal checking account transactions at our request, to see when her last social security check was deposited because we didn’t know if it would need to be returned to SS, and they saw the $20,000 withdrawal on the day she died and they didn’t say a damn thing! Long story short, 2 weeks later we found out about the theft and got proof (by gaining access to her online banking) and when my husband went down and asked for her bank records, they refused. Even though he was beneficiary of the accounts and successor trustee of her trust. We filed a police report and her boyfriend was questioned by the detectives and they sent the case to the DAs office and requested a warrant be issued for a multitude of charges and the DA declined to prosecute. It all came down to the fact that my MIL was not alive to tell her side. It did not matter that he was on recording telling my husband one thing and the detectives something else. Or that my BIL was witness to a phone conversation between the boyfriend and my husband after we discovered a second large theft from her account, and that during that conversation he changed his story several times and that one of his reasons was the same reason he gave for taking the $20k. We had lots of documented lies, like when we told him we had gone to the bank, he launched into a story about how she had no money left when she died and it all went to medical bills. We knew that was a lie, she was covered by insurance, she had excellent insurance. 2 days later when my husband called him and asked for all the medical bills for tax purposes, her boyfriend changed his story and said there were no bills because her insurance paid for everything! Didn’t matter that he owed the IRS tens of thousands and had his social security garnished because of it......didn’t matter that MIL had shown my SIL her bank accounts the day before the first major withdrawal and that she had never said anything about telling her boyfriend to take $14,000 from her checking account. That was the same week she had her trust and living will created and he was left virtually nothing. It’s a long convulsed story but there was so much evidence that she didn’t give give him the money and that it was carefully planned and yet no charges were filed.

i think at best unless your brother will admit to the theft, there’s little you can do.

Lots of missing information.

But before going into that. let me tell you that investigating financial crimes is not high on anyone's list. We had over $80,000 embezzled from our company and we were told we had to pay $25,000 to a forensic accountant to get proof. We have the proof in our hands. Had $80,000 worth of goods been stolen, the police would have investigated. It is incredibly hard to trace cash.

Has he taken the money since she she died, who while she was living?

How were the bills paid? If by online banking, then, no the bank will not do anything as there is no way to prove your mother did not use it to pay his bills. Even if she was physically incapable of doing so, how do you prove that she did not give him the PW?

If he wrote cheques on her account and signed his name, that is provable fraud, unless he has POA and signed as POA.

I echo Mstrbill's query:   If he didn't have signatory powers, how did he get the money?   Is it possible that your mother allowed him access? 

Even still, another issue arises:  specifically, w/o signatory powers, why did the bank honor the checks?   I think it was in error in doing so, and there may be liability there, although it's probably low enough that the police wouldn't get involved.  Still, there's a possibility of a complaint to the bank's management (national, not local).  

Did you ask whoever you discussed this with at the bank why they wouldn't accept a fraud claim?  Only they would know why they refused to accept such a complaint.

How did you discover this situation?  Did your mother have an online account?   Cancelled checks haven't been returned in years, so you'll have to have some proof if you do want to escalate the issue.

Techie raises some good issues;  How much approximately did your brother use of your mother's funds?  

Another very important question is whether or not he was providing care for her, and your mother made an arrangement (undocumented) that no one would ever be able to prove or disprove.     Where did her mother spend her last days, and with whom?   It's entirely possible she paid his bills as gratitude for caring for her.  

W/o being insulting, where were you when this was happening?  Did you find out about it in reviewing bank statements?   I have the feeling there are some more facts that could help explain how this happened.

And if you did try to get remediation help, I think the police or anyone else would ask the same question as to what your involvement was while this was occurring.     It just makes sense to figure out how this occurred. 

I will add though that I've personally seen a similar situation, when an elder withheld information from the intended Personal Rep of his/her estate, due to pressure of people who visited when the elder was alone and vulnerable.

Because your mother is dead, your brother can always say he had permission or that Mom paid the bills, effectively giving him the money. Just the fact that her money paid his expenses doesn't proof any crime. If the sum is relatively small, neither the bank nor the police department will want to spend the resources to examine handwriting on checks when your mother is dead and can no longer be victimized.

Because this is a fight between siblings over an elder's money, the police are much more likely to see this as a "civil manner" for you to fight out in the civil courts. If you can get your sibling to state in writing or in front of witnesses or the court that your mother gave or loaned him the money, then the laws of your state may allow you as the administrator of your mother's estate to collect the loans or view the gifts as advanced payments toward his inheritance. Although you may feel using Mom's money for his own bills was wrong, if Mom gave him the money then there's no crime, particularly if Mom was legally competent.

If his name is not on the account then he had no right to withdraw the money, Is it possible that your mother wrote the checks to him or that she withdrew the money and gave it to him. This would put a different spin on the entire matter.

How did your sibling access the money if his name was not on the account?

if I understand the question correctly, it sounds like your mom is being ripped off. Contact your area’s agency for the aging for help. They will not give you legal advice but can refer you to someone who can. Good luck.