We recently found out my eldest brother was "helping himself" to our father's money when he was also helping out with legitimate care expenses. Advice from the group about what to do or how to prevent this? I'm not reporting him to the police.

I hate to answer your question with more questions, but here goes:

Is your brother your father's power of attorney? Is there another family member who is appointed power of attorney and can step in and start managing your dad's money instead? If so, I would start there.

Have you confronted your brother about what has happened? You could tell him you don't want to go to the police and won't as long as he's willing to hand over management of your father's finances to you or another family member. 

Even if you don't want to report his actions to the police, you cannot allow him to continue handling your father's money. If he's helped himself to Dad's funds, he'll likely do it again.