Mom wants to gift me money each month for her care.

Your mom is a sweet lady. I wish all mothers would be so kind and considerate of their children.

Your mom offered the additional money to show her appreciation to you. Obviously, she wants you to have this money. You shouldn’t feel guilty about it. You haven’t asked for anything extra from her.

As far as your siblings are concerned. I bet that your mom hasn’t forgotten about them either. She sounds like a very generous person.

Count your blessings. I am sure that you are grateful for having such a lovely mother.

Your mother is 90 years old and a multi millionaire, meaning she's not going to need Medicaid. Allow your mother to pay you whatever SHE wants to pay you NOW, while she's alive, instead of leaving the money to you in her will. This will bring her pleasure and shouldn't bring you worry. If your sister or nieces or other family members catch wind of what mother is paying you, which is none of their business, then tell THEM to take it up with HER.

You may want to visit with an Elder Law Attorney who can give you advice on this matter. It is great that your Mom wants to pay, and when you think about it, that $10k is probably the going rate that one now pays in Independent Living [depending on the area where you live].

Hopefully your Mom has her funds in stock that is giving her a healthy dividend, thus funds that are used are being replaced by gains in the stock.

If your Mom is, let's say only in her 70's, then those funds need to grow for many more years. Hopefully she can live another 25-30 years. If your Mom is currently closer to 90 years old, then those funds should cover everything. As others had mention, draw up an Employment Contract.

Important, later down the road caregiving becomes more difficult as you are aging yourself. There will come a time when no amount of funds will make up the physical and emotional exhaustion. Outside help should be considered, or an Independent Living or Assisted Living. My Dad loved both when the time came :)

Simple.
Care contract drawn up by an attorney.
This needs to be legal because there are tax repercussions for your mother as the giver of the gift.
If she's a multimillionaire then she is self insured for her care, and need not worry about qualification for governmental care in future.

An attorney will draw up any gifting/shared expenses contract, or anything else that your mother wishes after examination of her for mental competency.

There is utterly NO REASON to mention this to any other family anywhere.
Were you to wish, with this enormous inflo of money to help your sister, then you might consider offering her the amount you can gift yourself tax free which would be 17,000 to any individual. Say a total of 34,000 to her and husband that they can put toward care of another sister. This would be very generous of you to do. IF that sister comes to you asking where, what, how you got this money you simply say that you and husband don't discuss finances that are private to your family.

We have been working with a tax guy and estate planner. So, gifting in our situation is fine. I’m more concerned with the amount each month. She originally said she wanted to compensate me because taking care of her stopped my earning power. So, do I take what I would be making in the marketplace and apply it to her? Or take what a private caregiver would be paid?

Fairness aside, even multi-millions can go PDQ when paying for nursing home care. Your mother needs to work with her lawyer and financial planner to make a good legal plan for spending her money because "gifting" any amount isn't it, she should be "paying" for room and board and any care received (and you need to be declaring this money on your taxes).

It's no-one else's business if your Mom decides to pay you. If you are confident that she has robust funds and will never need Medicaid, then it's mostly a tax issue.

"The 2024 gift tax limit is $18,000. For married couples, the limit is $18,000 each, for a total of $36,000. Gifting more than this sum means you must file a federal gift tax return in 2025. The 2024 lifetime gift limit is $13.61 million."

Source: Nerdwallet.com

So, you could have her gift the max and then have an employment contract with her. This makes her an employer and she will need to follow payroll rules like withholding and submitting a W2 for you. She can hire an online bookkeeper to do this for her. But I would consult with a financial planner, accountant or tax attorney first since no one on this forum are any of those things.

Have you checked in with your other sister to see if she needs support?

Please be aware that if your other sisters find out she is doing this, it will totally change your relationships with them probably forever. Your Mom could consider also making annual cash gifts to your other sisters while they are still young enough and can make good use of the money. Then your Mom gets the satisfaction of knowing how it helped them and there won't be bitterness and resentment towards the both of you. And, she can still pay you for your contracted caregiving.

Figure out your household expenses and divide that by the number of people living in the house. By household expenses I mean Insurance, mortgage, cable, gas, electric, garbage food...EVERYTHING. She would pay you that amount as her portion of the expenses.
THEN...
Get a "Caregiver Contract" done.
In the contract spell out what you are doing for her and determine an hourly wage. For example $30.00 per hour for an 8 hour day.
You should pay taxes on that amount.

If you are asking about "Gifting" a certain amount wait for one of the responses that I am sure will get into the "Gifting" in's and out's.