We have POA (health) for my mother in law, she is 99 and has dementia. Can we use POA to get SS address changed?

POA means nothing to many financial institutions. SSA is one of those. You need to get this form filled out to be your MIL's rep for SS.

https://www.ssa.gov/forms/ssa-1696.pdf

Depending on MIL competency or appearance of competency I would simply go down to the SS office with her or find out if it can be done over the phone but I don’t think so and change her address and banking info. As long as she sees this as a normal and smart thing to do there shouldn’t be a big problem. She moved to a different state, people do this, just assist her with making the legal change. This way you don’t even need to address the SIL or who has what sort of POA. If she doesn’t have a local account yet help her set one up, I would suggest making your husband or you a signer on the account so you can help keep track of it and help her pay bills or whatever with it but I might also suggest using just her SS and expenses out of it to keep things simple and on the up and up.

Social Security does not recognize DPOAs so having one doesn't mean a thing to them. Most government entities don't except them.

Saying that, I would go to a SS office (I don't like using the phone) and ask what needs to be done to have Moms money transferred using her now address and bank acct and u becoming her payee. (I may ask if SIL shows as payee) I would have a doctors letter handy stating Mom is incompetent to handle her affairs and that her son/DIL are now her Caregivers. If u find sister is payee, you may want to mention she is abusing it by only sending you a small amount of money a month. Its against SS rules to not use SS on the person it is for.

I would not tell SIL what you are doing until its done. That means setting up a bank acct and making sure funds are going into it. Then call her and tell her what u did. I may ask SS if there is there a way of getting an accounting of how Moms SS has been spent by SIL since u have had her.

If u run up against a brick wall, you may want to consult wit a SS attorney.

Tessabella66, to clarify: does this daughter in AZ have financial PoA? When you say your MIL has dementia, has she actually been formally tested and diagnosed by a doctor? Does the money that the daughter sends to you get deposited in an account with your MIL's name on it? Thanks for the additional info.

Also, if you don't make any headway with what Alva suggested, I'd spend the money and consult with an elder law attorney who has experience with financial abuse/fraud and (depending on the outcome of that meeting) let the daughter know she may be the subject of an investigation.

No. Social Security has separate rules. Is your mother competent? If so you can assist her in putting through a change of address, and in going to Social Security to redirect deposit into an account in your area. If your mother is NOT COMPETENT you should go to Social Security office and explore how to become her "representative payee". If you become this, you will need letters of incompetency from your mother's doctor.
Social security will run an investigation on you. You will then have yearly forms to fill out as to what is happening with her SS check; you will need to keep careful records.
Even if your mother is mildly demented, she can appoint you as her POA for financial; it is only necessary that she understands exactly what she is doing when the lawyer speaks with her. If Mom is not competent then you may need to apply for guardianship.
THIS is your mother's money. She may someday need it. You cannot insure that she will always be with you and in your care. As was stated below, the use of these funds by the sister is endangering your mother's finances. You honestly need now to speak with an elder law attorney.
Good luck. Go to your local Social Security office. They will tell you exactly what you need.
You cannot use online as your mother has likely never signed up for My Social Security. You cannot do it FOR her; that would be fraud. So don't try that.
You can also explore with the lawyer setting up a trust account for Mom's funds, which you handle as her trustee; again, careful record keeping, and Mom needs to know this is being done.