I have worked with foster kids. Foster kids are more complicated that regular kids. And there is usually a honeymoon period of about three months, so if these kids stay with you, don't be surprised when the other two girls start acting up. It WILL happen.
However, this girl is only 7, and the instances you are describing are not that terrible or unusual.
Usually when kids whine you say something like, "I don't hear whining," or you ignore it. Or you say something like, "You will get money when you lose a tooth."
If she says something about wanting her mommy, you mirror her emotions by saying something like, "It's so sad that you can't be with your mommy right now." And give her a hug. Foster kids are very attached to their parents, no matter how terrible the parents have been to them. It doesn't matter when she is expressing this loss, the loss is real, and devastating to them. You need to comfort her when she expresses missing her mother, no matter why you think she is doing it.
I have to agree with the others that you are probably not equipped to deal with foster kids. Sorry. I understand why you were given the kids, because it's so hard to find homes for foster kids that social workers have to use whatever homes are available, especially when trying to place three siblings together. I have known kids that went through 30+ foster placements.
You will have to be very patient. You have unrealistic expectations of any kids, let alone foster kids. I assume you've never had your own kids, if you think a 7 year-old whining about things not being fair is unusual.
No child should have to be removed from their parents. This is not her fault, so you need to go out of your way to be understanding.
And PLEASE, don't expect her to be "appreciative." She's a little girl who has been taken from her mother. You are expecting too much.