Is there a process to have a third party help control or guide my parents accounts and monetary decisions?

Are they bankrupting themselves by giving money to your brother? If they are competent, there's really not much you or anyone else can do. They have the right to make decisions, even if you don't agree with them.

Perhaps they have an agreement with your brother that these loans are an advance on his inheritance?

If you are in Texas, and I got that idea from your nickname, there is a brochure online regarding guardianship and alternatives. It outlines the responsibilities and pros and cons. It can be found at:
http://www.dads.state.tx.us/news_info/publications/brochures/pub395-guardianship.pdf

I hope it can be of some help to you, even if you don't live in Texas, some of the information might be the same.

Can you express your concerns to them in a family meeting, with your brother there? It seems like an unwise thing for them to be doing, BUT if they are competent, sadly there is not much you can do, since they are their own POA before you get to be it (well it depends on exact details of their particular POA document). A lot of seniors do make huge money mistakes and it can be difficult to see happen. Sometimes seniors will help other family members "more" than we feel they should....but as long as they are competent we can't prevent it. IIt's their money, they can spend it as they want to, they can even bequeath it to their cat if they want to. Its not your money until they both pass, assuming there is anything left and they leave it to you. Its also wrong for a POA to try to influence the principal in a way that would personally benefit the POA, so tread lightly.

If you are afraid that your parents are impoverished them elves to the detriment of their present and future caregiving needs, I would take several steps.

I would discuss what they are doing with their doctor. Is this a sign of cognitive decline? Maybe; a lack of ability to appreciate consequences can be a symptom.

I would talk with an attorney about what your duties as POA entail. Are you responsible for them having funds to care for themselves down the road?

I would talk to my parents and tell them that I can no longer hold their poa, since there will be a long look back at funds gifted, and that I might be held responsible.

You say you brother is harassing them. Is he threatening them, intimidating them? Call their lawyer and ask for his advice about this, as well.