My brother lives in Mom's house; is there some rule or protection so that my 70-year-old brother may remain in what is his home?

Brother should be fine for the reasons JoAnn pointed out.
House as his existing address important too to clearly establish he’s been there 2+ years prior to her going into MC so that should Medicaid application be done, he will not be viewed as a person needing to pay mom rent. Bro was there as live in caregiver, so no rent needed.

Do NOT transfer ownership of the property. It will place a penalty - done by # of days but based on tax assessor value- should mom need to apply for LTC Medicaid. All stuff like this gets recorded at courthouse and dovetails to state database. State can look back as to 5 years of assets from date of Medicaid application.

I would be concerned about property’s affordability over time.
Right now it’s moms house & probably has her homestead exemption & other senior perks. As long as she’s owner, all that should stay in force. And if her LTC insurance is covering her MC costs, mom can still have $ - like her SS or savings - to pay property costs. But should she go onto LTC Medicaid, she will have to be basically at impoverishment point at 2k max in nonexempt assets & will have to have her SS$ go as her required by Medicaid copay to facility.

ltc Medicaid = no more house $.
So can your 70 yr old Bro. on his own afford all property costs along with his living costs? If mom has been basically underwriting all costs, this is going to be an issue eventually. As moms DPOA, you really need to get a fix on her $ and exactly what costs have been like for ‘18 & ‘19. And Bro needs to look at his $ to see if he financially alone can afford home. In my experience, your mom continuing to pay taxes & insurance & monthly yard guy will be ok for Medicaid application review as to where her $ went while her LTC insurance paid for care. But her paying for utilities or buying anything “house” could be a problem. She resides in a MC; house isn’t empty / vacant with minimal utilities running (like security lights). Bro should imo be paying utilities & replacements (like new heater, refrigerator) as he’s actively living there & not paying rent.
Can he afford this? And for an undetermined period of time?

Medicaid by & large allows them to keep their home as an exempt asset for their lifetime. Again if on LTC Medicaid, they have no $ to pay property costs on old home in their name. So family will need to pay & do so now and till beyond death; & will have to deal with an attempt by Medicaid /MERP via a lien or a claim. There's all sorts of exemptions, exclusions to MERP, plus your states property & probate laws can factor in. But all takes time & $$$ & runs a certain amount of risk. Give all this some thought.

Also, If possible, try to get Bro’s name added onto utility bills & property insurance. Should something happen - like hail damage or gas leak - if his name is on account, he can authorize stuff. You as DPOA can’t as easy as utility co / insurance may actually have to have your dpoa on file or want to speak with mom to do squat.

Also I’d try to quietly find out if your state does the all heirs equally for MERP. So each heir will need their own exemption or exclusion. Like say it’s 150k home with 3 heirs as per valid will: 68 yr old caregiver daughter, 65 yr old son and 60 yr old legally blind son. All equal heirs. 68 & 60 can get exempt but 65 yr old won’t. His 1/3 share may have to be dealt with to settle a possible MERP lien or claim. If your state does this, you’d need your own plan for dealing with MERP. Bro has caregiver exemption.

If your on LTC Medicaid parent wants to keep their home, it can be done. But DPOA (& future executor) need to have it makes sense to do, cause if other family members don’t do whatever they promised, it’s all gonna fall on you. Could be manageable till forever or major problem when taxes due. Really try to get with Bro to go over all this & soon. Good luck too!

Don't transfer anything. It could effect Moms ability to get Medicaid.

I think as Moms caregiver he will be OK. Eight years he has proved residency. It has to be his main residence. He should not have a house somewhere else. I am basing my info on Merp in NJ.These were questions asked on the form I received after my Moms death. She still had a house. He may have to prove he can pay taxes, utilities and upkeep. Be aware, that if he is allowed to live there, when Mom passes a Medicaid lean will be placed on the home. He will be allowed to live there until he leaves, sells the house or dies. At that time, the lean will need to be paid.

I am just giving you info so you have an idea. Each state varies. I would physically talk to a Medicaid casework to see if this is how they handle it.

Go to your local Medicaid office. They should be able to answer your questions. Depending on your state he may eligible for a child caregiver exemption. Also contact your county Area Agency on Aging. They may have a Medicaid specialist or be able to direct you to a less expensive elder care attorney. The social worker at the memory care may be able to help with Medicaid application.