Daughter-in-law is presently POA. Her husband (son) says he is in control. Sister would like security of another person being added, is that possible?

Since your nephew is not listed as his mother’s POA, he legally has no say whatsoever about his mother. Your sister can add a a close friend as POA, but the daughter-in-law will be the primary POA and the friend will be the secondary POA.

Your sister being able to understand what is going on will need to be assessed by the attorney who will be revising the PoA arrangement. I agree with others that adding more PoAs will make sister's care more complicated because people are people and aren't perfect... they disagree and fight.

Also, it is NOT a good strategy to assign some as PoA who is the same or similar age as the principle. That person will get old and have problems, too.

And, if you're getting all your info from your sister (who you said has dementia) then she may not be giving you accurate information... especially about the house issue. People with dementia develop paranoia that can get progressively worse.

Finally, who's to say another non-relative PoA won't want the house, too. Elder financial abuse happens all the time. Sister could consider adding an attorney as another PoA, but again... I wouldn't.

I've held joint POA and will never do it again. The grantor of the POA generally thinks it's a great idea to have that check and balance. It isn't. There are so many minor details with everything that must be done, and it's easy to hit a snag. The other POA can get into a snit and start with the arguing, and that can be a full-fledged fight before it's over. Furthermore, as happened in my case, the joint POA started having mental issues and still does. What can you do in that case? And they have a serious drinking problem besides? Yikes.

One POA. Choose wisely and well, and name a secondary POA to whom duties pass if the #1 POA has to bow out.

I would revoke DILs because of the son. Then I would assign someone else with a secondary who can step in when the primary can no longer do the job. I so hope she has a Will. Me, I would make sure that the son did not get the house with his attitude.

If sister is still legally and mentally competent she can do whatever she wants - she can add POA, change them whatever.

However, here is the "rub" so to speak.

It is generally accepted that the POA is one person that you feel will act on your behalf, the way that you would if you were capable of making your own decisions, that you trust to act in your best interests.

Sounds great.

When you start naming two or more people - to be CONCURRENT POA or SHARED POA - rather than SUCCESSIVE POA - in other words named in order so that if POA #1 either can't or won't serve, the next person in line becomes their successor - you could unintentionally be creating problems.

Initially - when we first looked at FIL's POA documents - we thought that he had left DH and SIL as joint/shared POA. Which actually would have been ok for the most part because THEY agreed on his care. With one glaring exception. HER DH. He had ulterior motives that, while not nefarious, were self-serving. And not in FIL's best interest. And he holds very strong sway over SIL. That when he thinks is needed - he leans into VERY HARD.

So if your sister is going to redo her POA - I would strongly encourage her to consider the way she has it worded. I wouldn't suggest that she have them as joint POA if she suspects that her son is going to exert undue pressure on the other person/people to do things his way. Whomever is in that position needs more power than him in that case. If that is the case she should name them primary POA, and then DIL as secondary.

If she's not legally or mentally competent, it doesn't really matter. Nothing can be changed.

Take your sister to an attorney. That attorney (hopefully the one who made original POA) will examine her for understanding and competency to change her POA and he will give her various options for how to do that.
Recognize that have TWO operating at the same level of POA rather than as alternative when one cannot function, is VERY Confounding and often an utter mess of argument.
In my humble opinion that doens't work well.

Get the advice of an Attorney.
Only your mother can revise her POA.
It is a difficult mess in that accounts in banks and other entities are already set up.
You need to know
A) if mother still legally competent to do this
B) if this is at all a good idea
Get options legally. Good luck. Hope you will update us.

These family issues are so complex! If she made DIL POA, then so be it....it's legal and takes precedence over blood. There must be a reason she chose DIL over son! She should be able to add someone to POA. I suggest that they be added as a back up person, not with equal say as DIL because that just complicates things if they don't agree.