With Durable POA can I stop parents from getting Reverse Mortgage?

It really depends upon the kind of POA they have, the wording and their competence.

I hope you have explained to them that they will be disqualified from receiving Medicaid assistance due to gifting, unless they have a legally promulgated "caregiver contract" whereby your brother is being paid for his services to them.

Have you made it clear to them (and resolved it in your own mind) that YOU will not be stepping in when they need more care?

If they are competent, they are free to make dumb choices and suffer the consequences.

You absolutely can try to stop a reverse mortgage by telling your parents directly that if they go ahead with it that you will resign as their DPOA and be unable to help them with any of the consequences of their decision. That is the line you draw in the sand and if they cross it then you walk away.

At one point we went through a similar struggle with my in-laws when my mother-in-law was still alive. My husband had DPOA for both medical and financial for both his parents. And one time when my MIL didn't like my husband's advice, she snipped at him "Well I can cancel your DPOA" to which my husband said "Go right ahead!" The look on his parents' faces was priceless. They treated him better after that.

Your parents have an unhealthy relationship with your brother. They are co-dependent. Time to put your big girl pants on and tell them how things need to be or else you cannot help them because you will not enable their bad decisions.

Why did your parents feel they needed to support your brother. And 5k a month? Thats 60k a year. Now they are 85 with only their home. If they pass still owning the home, even if its sold and you share the proceeds, it won't last him long. So now he is on his own in his 50s/60s? Did he ever work. If not no SS or Medicare.
I hope he doesn't think you are the milk wagon.

Are they competent to make there own decisions, if so, then no you cannot tell them what to do. You can suggest. Now if they have both been diagnosed with a Dementia, then they are not capable of signing a contract.

Sorry to say, that they supporting your brother may effect them being able to get Medicaid help in future. Its considered gifting.
Brother needs to be told the gravy train is no longer. That he has taken all he can. The house is all your parents have if they need future care and reversed mortgages are not always a good fix.
I don't live off of 5k a month.

Is your brother disabled in some way? If so is he receiving SSDI?

Yours may be a question for an elder law attorney or Medicaid planner for their state.

If he tries to get them to risk their home, I would file a police report for financial exploitation of vulnerable seniors.

Is there any particular reason your parents won't say no to your brother?

Your brother has received support from parents for his entire life. Hence, for whatever reason, this is what your parents have elected to do whilst in full control of their mental faculties.
Having DPOA gives you no authority until your parents are deemed mentally incompetent and until then any determination is entirely at their discretion. And from there you need to consider objectively what your parents would wish to do under the circumstances (and not how unjust his favoritism has been) since your role is to act as they would themselves - even if reckless, even if to your own detriment.
Sadly, avoiding conflict of interest is your moral obligation although priority must be toward providing your parents with safe and supportive care for the duration.

With Durable POA you can only intervene if your parents are declared by at least two doctors as being unable to manage their own affairs.

Read your document! When you can act and what you can do are spelled out in each individual DPOA document. While many only kick in when your parents are no longer competent, others are not written that way.

I don't know if (or under what circumstances) you could override their decisions, but each POA document has its own provisions.

Please tell us more. From your profile: "Hello, I'm struggling with how to deal with a parent I am in big conflict with."

What's that all about?

Tell us more about your brother.

If they are gifting your brother $5K/month, there will be a big problem for Medicaid eligibility down the road. Or are you going to be their caregiver? What is it that you do for them now?

What are YOUR plans for their future, because it sounds like they don't have any (other than possibly assuming YOU will be the caregiver).