My wonderful mum and MIL played an important role in helping with childcare when my dc1 was born. We asked them what they wanted to do, rather than tell them what we needed. This meant that we did a mix - one day with grandparents, and some paid childcare.
When my dc2 was born my mum was older and less able and I spoke to her about using only childcare at that point. My in laws continued to help because they are younger.
I would never have expected to rely on family solely for childcare unless they had offered and wanted to.
Please don’t feel bad - you aren’t in any way unreasonable. She should have had a conversation with you rather than just expect it. Things change, you are older now. But not only that, while I’m sure you love your grandchildren, you are also entitled to a life beyond caring for them.
Her eldest two are in school therefore they will need to find a way to pay for childcare for the baby.
Have a think about that you want to offer - zero days, one day, a half day for the baby etc. And do this before you speak to her so that you are firm in what you feel you can manage and want to do.