Actually the POA ( power of attorney ) person has or had all the receipts of expenses.
Your husband chose his sister as POA, which leaves you out.
It is up to him to ask for a monthly accounting of how much is spent and how much is left monthly. If he is unhappy with her choices he can go to his attorney and revoke her privileges and make You POA.
He evidently has not chosen to do so.
You either are not considered trustworthy, or he considers her better suited in money matters. He could request her to go over the monthly statements and balances with you. Yet he evidently does not?
You are powerless to change anything unless you feel she is defrauding your husband. You would need proof he is not receiving food, medical care etc. You would then need to consult with his attorney and with your husband if he is in his right mind.
Your disabled daughter has no place legally in this business arrangement.
You have basically 2 choices stay or go, again unless your husband is being defrauded.
This is from what I know in the US.
Social Security can only tell him his benefit amount, not how much is spent and for what. The bank can only account for that, assuming she deposits the SS check and then is on a joint checking account with him and then writes personal checks on that account for his bills, drugs, food etc.
The first person You must consult with is your husband! Without his consent no agency is going to answer your questions. The bank(s) can mail him duplicate statements if he inquires.
You are out in the cold, especially if he wills everything else to his sister upon his death. Better you find out from Him now than later!