My father passed away about a week ago. My sister had her name on his banking account. she spent most of his money.

If she was a joint tenant with right of survivorship on on his account then it was her money as well as his during life and passed to her automatically at his death. That said, if you would be able to prove that her name was merely on there "for convenience" and it was all his money and she was on there as his "power of attorney" then you may be able to argue against her. Achieving that is very difficult and would require that you hire an attorney.