Oh good grief, she is being cagey on purpose.
The painting is not yours, she is just painting a child, and wants to use your child for it.
She is NOT painting the painting... for you.
She is painting the painting, for herself, for her own use.
Then, whatever she does with the painting, is up to her.
It is not, yours. Nor is she even trying to get YOU to buy it.
She just wants to also use your daughter as a model.
She is full of it.
This is a "hobby" of hers???
Yet, she sells these paintings. So, she must have a business license????
IF you do NOT want her to sell a painting/picture of your daughter...then the answer is: DO NOT LET HER PAINT your daughter.
Simple.
Then, if she works off of snapshots, who then gets to "keep" that snapshot of your daughter, that you provide to her? Or, is SHE also going to take the snapshot of your daughter, too, for her painting????
Again, this is HER painting. Not yours.
But it will be your daughter, that is painted.
And no, you do not have a say in it, per how SHE does paintings.
BUT this is a child, and she would need your permission to do it, I guess.
There is nothing in it, for you. If you allow her to paint your daughter.
And, she IS a "business." She is, painting and selling, paintings... and she sells her paintings somehow and somewhere. Or perhaps even online. Which some "artists" do.
Once a person makes $600 or over in any income, they have to claim it on their taxes. Even if you are a babysitter.
Again, I personally, would NOT have my daughter painted.
The woman sounds cagey and I would not trust her.
Then, make sure that she or her sister, does not take any photos of your daughter themselves, because then, the woman could paint your daughter. Even if you do not provide the woman with a photo yourself.
Bottom line is: you do not want her to sell the painting to someone else, because it is a picture of your daughter. Then, you also do not know, IF you want or can buy it yourself. Due to the cost, which the woman will not specify.
Then, you do not even know, what her paintings are worth because, there is no way to estimate the value of her paintings.
And you don't know what she even does with her paintings, if she sells them or just keeps all her paintings just accumulating in some storage since she claims to be a "hobbyist" at it, or if she shows them at art shows or at sidewalk flea market type places and sells them there.