Is a financial power of attorney in one state valid in all states?

Consult with a local attorney that is in the state where she will sign it. There could be differences in how many witnesses are required or some other details.  While you're there, I would explore her Healthcare POA too, and an Advance Medical Directive.  

It all depends on State laws. Your State might recognize a POA from one State but not another.

If you think your Mom can still understand what is a Power of Attorney, I would have a new one drawn up in the State where she lives. And as Sunny had mentioned above, also a new medical POA, Advance Medical Directive, and even a new Will which reflects the State laws of PA if that is where Mom is now living.

I would highly recommend you use an Elder Law Attorney. And while you are at the law office, update your own POA, etc. to reflect current laws.

Marg - I live in New Orleans & was DPOA for my mom in TX and lots of folks in NOLA area have camps & 2nd homes in MS or retire to MS. Here's my thoughts based on all this: 
- for banking an out of state DPOA worked with no problems as I also maintained a bank account in my name at moms bank (Frost which is TX only) AND I was a signature on her account and it was POD to me & all done well before she got dotty.
- If there is a banking group that is in your state & MS I'd look to have her bank there & close out others. 
- MS revised its POA conditions (MS HB 468), so if there's a old POA done before 7/13, it's probably got issues in being accepted. Mom needs a new one done in MS with MS notary seal if she's a MS resident. Also a lot of places (banks, ups stores) that in the past in MS that would notarize documents will no longer do them for anything beyond vehicle transfers or other simple stuff. There was a lot of illegit paperwork shenanigans after Katrina & Deepwater Horizon, so notaries gun shy. Long story short, you need MS atty. Btw there's just a handful of NAELA level atty in MS, you may be better off finding one in LA that also holds a MS license.
- DPOA can be registered at moms county courthouse. Most states don't do this at all. But MS does. If you anticipate problems with family continuing, your going to want your DPOA registered at the CH. If your mom still has a home I'd suggest when you go to register the DPOA that you also pay for a copy of all documents attached to the PPIN. In MS, all are pretty cheap, like $ 8.00 for a Warranty Deed. I'd be concerned that brother has her home as collateral for his borrowing.... So you want to see if any judgements or any other "clouds" placed. 
BUT
- the bigger problem still remains that IF mom still has access to her checkbook or debit card or can go to the bank, she can continue to give $ to whomever. Being DPOA does not keep that from happening. To absolutely keep that from happening you would need to file for guardianship. This site has oodles of great posts on guardianship adventures. The biggest hurdle imo will be that as mom is a MS resident then the MS judge will want the guardian to themselves be a MS resident. Which you are not but your moms NAELA atty could be. Or the judge appoints an outside guardian from a vetted list judge has at the ready. Or family puts on a show and gets appointed.

If your deadbeat family (we all have them, my moms was a worthless nephew), just will not stop asking for and getting $ from mom, your choices are stark..... Probably moving your mom to your state would be ideal as you can oversee both healthcare & finances with less drama.

If you anticipate Medicaid will be applied for there will be issues with whatever $ mom gave to others. MS Medicaid will want 3 years back of all financials initially and if any issues will require a full 5 year lookback and within 30 days from letter of request. If you needed to gather these could you find & copy all within a week or weekend or two??? That $ to brother & niece will be viewed as gifting and will snag moms approval till forever as there will be a transfer penalty. It sounds like they are draining her $ 1,100 each month. If this has been happening the full 5 yr period of the lookback, it would be $ 66k. Roughly 415 days of Medicaid NH ineligibility which somebody would need to private pay for her NH stay.

Moms atty will have to be very proactive in all this to get her hopefully past any gifting / transfer penalties by MS Medicaid. MS has really tightened up on SNAP & Medicaid with rolls down about 30-40% and this before whatever happens with the ACA repeal. It's not pretty as MS is beyond poor.

None of this is simple. To me - unless you can go all bad witch with errant family - you are going to need an NAELA atty & ASAP to protect moms best interest.

My parents lived in PA, I moved them to MD with me. Never had any problems with the POA.. anywhere!

When I moved my mom here to Minnesota from another state, I had to get a MN POA. They would not recognize the one from her home state. I imagine each state is different.

I would visit with the elder attorney first. Assuming mom has the capability to make these decisions. If not, you would have to file for guardianship which is much more expensive than a DPOA. Good luck on managing moms money. Remember that as long as mom is competent, she has the right to give away her money and as her POA your job is to carry out her wishes. However, should she need medicaid in the future this gifting will prevent her from qualifying as there is a five year look back by medicaid. So it is a fine line. As long as mom is competent she can always change the POA from you to sister or brother or niece or just cancel it. Most likely this will be explained to her by the attorney.So the person you may have to say no to is Mom when she needs to borrow money, if that is what is going on. She is basically having you subsidize brother and niece if she gives them money and then you have to support her for the rest of the month.Many people think that when someone gives them a POA they automatically have the right to control the principals assets, not so. You are their agent to do as they would wish until they are declared incompetent and you are then able to exercise that portion of the DPOA that allows you to act for her. I'm surprised the brother and niece wait until mid month, most of the moochers I've known show up the day the check hits the bank account.

Most states honor POAs from other states - it is called "reciprocal" or "reciprocity" - provided your POA complies with that state's requirements. I never had a problem, even when I was in MA, the POA was executed in NH, and the grantor of the POA was in Florida. The one place that you WILL have problems regardless of the origin of the POA or if you have a guardianship or conservatorship, and that is with the federal government. If any of her income is from the VA, or any other federal agency, they insist on having the benefitted party sign the documents - even if they are in advanced stages of dementia - but will not accept a court ordered guardianship, which you have to go through h_ll to get. It is insane, and when someone has dementia, they'll sign anything for anyone - but that is all the VA accepts. I wouldn't worry about it, unless you are planning to move? If so, maybe make sure that you have a POA acceptable in both states - or do 2, one for each state, is what I would suggest.

Also, if you go to an elder law attorney, make sure they have training in financial planning, especially medicaid planning. One mistake could make the difference between her getting the help she needs, or having to wait many months because a financial transaction ran afoul of the medicaid 5 year lookback rules.

Ask an elder law attorney.

Wording from state to state can be different. The one that I had done in CT was not legal in VT because of the differences in language. I had peace of mind knowing that it was done in our new state.