Can a person with supplemental social security have control over someone's financial assets?

I would say if Mom tries to change bank accounts SS will question it. Checks are now direct deposited. She would have to prove that grandmom is no longer competent to handle her own finances and will need to become Moms payee. SS does not recognize POAs.

I would think, as any other government agency, that SSI requests bank statements when renewed. Co-mingling her and Moms money is not wise. She could lose her SSI and Medicaid if she gets it. And if Gma ever needs Medicaid it will cause problems. So, its not wise doing this.

Your mother should not do this. Not just for the reasons already described which are 100% accurate but also because yes, it can cause her to lose her SSI. SSI is income based. Your mother cannot have the retirement income deposited in her name because it will bring her monthly income over the limit for SSI eligibility. She can be added as a signer on your grandmothers bank account & that will allow her to use the account to pay your grandmothers bills.

Bad idea to co-mingle funds at all, let alone given the POA's financial circumstances. She will find it much easier to keep clear, accountable records of your grandmother's income and outgoings if she opens a dedicated bank account strictly for that purpose. I would expect the bank to be able to guide her about the best way to do this with power of attorney - the account will be in your grandmother's name, but with your mother as POA being the authorised signatory, I think that's how it works, roughly.

It is illegal for a POA to enrich themselves with the assets of the person they are acting as POA for. That is the first no-no.
Even were it legal to do this, it is morally wrong.
Even were it not illegal and immoral, your mother would almost certainly lose supplemental by upping her income in this way.
It sounds as though your Mom doesn't understand AT ALL what she is to do as POA. There are entire books written about it, but were I you I would encourage her to seek an hour of time with an elder law attorney to explain her fiduciary duties to her; THIS she can pay for on her Mother's income, because it involves learning how to handle her mother's assets. Do know that Fiduciary responsibility includes careful and meticulous record keeping so that your Mom can answer for every single cent spent out of her Mother's assets.

It may be possible but I'm pretty sure it's frowned upon, if not downright illegal. Her mother's money is for HER care, now, whether she wants to 'help out' with the household expenses is one thing, but I believe that she should keep funds separate and with complete transparency.

People think POA means an income. Really? Being someone's POA is a pain. Just MHO.