What rights does a "significant other" have to make care decisions without a POA?

He has no rights. The hospital can even refuse him visitations.

this is one of the reasons Gay Rights movement fought for so many years to have the right to legally marry.

She can become a ward of the state.

Her boyfriend can do nothing. But sis can and maybe has had a new POA prepared, and then, only if she is considered competent can she sign it, then only if she is incompetent can boyfriend use it.

It all hinges on whether your sister can be judged mentally competent to make her own decisions and whether she understands and accepts that her son will fail in his duties as her POA. Because you asked this question, are you planning to take matters into your own hands? Her live-in really has no obligations to care for her if nothing to that effect is legally on paper and binding. He can just walk out if things become to stressful for him. If you see this on the horizon and want to become involved, investigate who drew up the original POA and see if they can help.

(Looking this over, I see I may be a little bit grumpy this morning. If I offend anyone, I'm sorry in advance)

I'm wondering a little bit what's actually happening here? Your question is about what 'rights' her boyfriend has if he's not the PoA. You say you don't feel it is your responsibility. You say she has given her son Power of Attorney but he is not 'mentally able' to do what needs to be done. Sounds confusing.

Is her son cognitively impaired? struggling with mental illness? or do you just mean that he's having the customary amount of emotional turmoil over a parent's decline? or do you mean he's not doing what you think he should?

You say she is 'pretty much on her own if the boyfriend and/or son walk away' ... okay, but that's true for nearly everybody: if the closest live-in family walk away, we are on our own.

You also say she is refusing to go to AL and that the boyfriend seems willing to care for her.

So what's really the issue here?

I don't think I understand why the son, your nephew, is unable to use his power of attorney. When you say he is "mentally unable" - for why? In what way?

Surely, if he had a known, well-established learning disability, or a debilitating mental illness, your sister would not have asked him to accept this responsibility; so what's the problem?

Her boyfriend can refuse to care for her and move out.

What is your sisters plan for getting care for herself? Is she competent enough and wealthy enough to hire in-home caregivers?

"What can her boyfriend do, as she is living with him and the care will fall to him?"

He can, if he so chooses, provide in-home care for as long as he can and wants to. No one will take that away. However, without POA (medical and financial), he likely will run into issues. There are forms the hospitals/doctors use that she might be able to sign to allow him at least access to medical care, but probably not decision making if she needed extensive treatment. Without DPOA he technically can't manage her finances. If they have a shared bank account and her income (retired?) is direct deposited, he *can* use the account to pay bills, however there are rules about that (esp SS)- so long as no one questions anything, it can work, but if he decides to walk away, what then?

"I can't take care of her and so far, he seems willing to do it. Time will tell. I really don't feel it is my responsibility. I have other siblings and they are unable to take responsibility either."

Although it still means participating and some effort on someone's part, please understand that having DPOA and MPOA DOES NOT REQUIRE the person to provide the CARE. You handle the finances and medical decisions, but you do not have to care for her. If one of you decides you can do this, at least temporarily, and you find an attorney who will test/accept her ability to sign, the old POAs can be negated and new POAs added. At least for the short term someone can be "in charge." POAs can also be declined at any time.

If no one takes on these tasks, someone will have to ensure that the state "takes over" and assigns guardianship. Once the state steps in, you have no say in anything, how her funds are used, what happens to any of her assets, where she lives, how she is cared for, what treatment she gets.

Also note (many don't understand this!) - having any kind of POA does NOT allow you to forcibly move anyone. You can choose a place, sign paperwork FOR the person, arrange payments from their funds, etc, but oftentimes trickery or a medical emergency is needed to facilitate the move, especially when they refuse (been there, done that with our mother!)

Her boyfriend can be her caretaker as long as he is willing to do that. He has no legal obligation to do so nor does he have any legal rights to make decisions for her or handle her medical or financial affairs.

If she is competent she can revoke son's POA and designate boyfriend or somebody else.

It appears none of her children beyond the "incompetant" son are able or willing to take on her care.

You do not say what is wrong with your sister? If her son has POA and is unable to perform those duties her significant other can do this if she withdraws appointment of son and assigns significant other. I am assuming they are long with one another and in trusting relationship. She can assign him to quite a few things, just as she could, in fact, assign a trusted friend. If they have not already done so, they might want to see an Elder Law Attorney before becoming LEGAL significant others; my Partner and I are "Domestic Partners" in the State of California. Federally that is not recognized. So we can file together for income tax in State of California, but not in Federal system yet have to send papers to feds to say we can legally do this in California and it all gets very complicated. Where it REALLY gets complicated is that it does in some states (check their own) "marry" you for all intent and purpose, say for medicaid qualifications and so on. I would not do it unless there is great benefit for you in some way, because getting out of it amounts to same procedure as for divorce and would need to happen for division of assets, and etc. Any time you have the Feds looking at you one way and the State another it isn't really a good thing.
Anyway, they should definitely go together to an Elder Law Attorney. Again, you do not say how disabled you sister is--that is in what manner, for if she has now gone into dementia she can no onger change the POA. Having a mentally unable son assigned would be a complete mess if they are NOT married or Domestic Partners, in so far as guardianship. I am saying, if someone else in family wants guardianship they are more likely to get it.
So key things here.
1. In what way is your sister disabled so that she cannot manage her own affairs
2. What are the LEGAL ways she and her significant other are attached
3. How long has she been with that person/how trustworthy does family she him to be
4. What other family is involved?
5. In what way is son "mentally" unable? What is his legal diagnosis? How did he get appointed if mentally unable?
I would suggest a trip to an elder law attorney.
I see below you said "not competent. That means she CANNOT change her POA to the boyfriend at all. You may eventually be looking at guardianship by the state. You don't want it. The boyfriend can't get it, and the current POA apparently cannot do it. It is kind of a mess. So very sorry.