Another idea. Perhaps you and your sisters could arrange family time which includes you looking after the older kids while your mother looks after the grandkids. You could take the older kids somewhere and leave the grandkids with your mother. Or perhaps you could break up your mother's kids into smaller groups for different activities so that she doesn't have all the children and you at the same time. I don't know the ages of her kids and so I can't think of ways to do this.
I've also been a foster mother, once with two girls but the rest of the time with only one. Foster children have different and in some ways greater needs than most birth children. They are definately more difficult to parent. I couldn't manage the two let alone 5. She needs a break. Could you and your sisters arrange for respite care for the foster kids and have your mother visit you at your houses? There might be respite care available thru the agency which placed the children with your mother.
I agree that you definately need to arrange a time to talk alone with your mother to let her know how you're feeling about this and try to problem solve together.
I understand that your mother is necessarily too busy with her own children and the household responsibilities this entails to be available for much interaction when you visit. But if she knew how much you wanted to spend time with her in a more personal way they might be a way of arranging that. My first thought is for you to visit when the kids are in school, if they're in school. Perhaps help your mother with chores while she plays with the baby.
You are a new mother with new feelings of love for your child. You naturally want your mother to be more involved with your baby than she's able to be at this time. It's true that you and your baby are having to give up much so that those children can have the nurturing and care that they need.
I understand why you'd be hurt and resentful. I suspect your feelings are getting in the way of a better relationship with your mother. She feels your resentment even if she hasn't acknowledged it. I think that you will have a better relationship with your mother if you can talk with her and find a way to accept her children. She didn't give birth to them but they probably still feel like her children to her, especially for the ones she's adopting. It doesn't mean that you and your daughter are less important. It probably means that she also feels responsibility for these children and hasn't found a way to include you and your baby.
I have confidence that you and your mother can work this out.