Siblings taking my father's money.

My elderly father has short term memory loss. I have access to his account to track his expenses, but am finding that he is giving large sums of money to my siblings, who are willing acceptors of the funds, who I believe are taking advantage. He doesn't realize how much he's giving away, and refuses to turn over his funds entirely to me to manage. Has anyone dealt with this or something similar, and is there anything I am not thinking about (aside from sibling interventions, reporting elder abuse or making him take the competency test {which I actually think he will pass because he's quite competent with everything aside from short term events and situations})?

Help!