I think it is a difficult one. I can see why you feel it is unfair- I suppose in some ways it is because your sister has had a lot of support from your parents which it looks like you won't be getting. It's a shame, but maybe there is a good reason for this. Circumstances change and what can be offered is not always fair to everyone.
However, look at it from your parents' point of view- can they realistically look after all the grandchildren most days of the week? I think that it is unrealistic to expect them to do so. So it follows, therefore, that you are expecting her to change the arrangement she has with your sister. Maybe she doesn't want to suddenly withdraw a long-standing agreement between them to suit you? Maybe the fact that they are away at school all day now is what allows her to cope, and she feels she will struggle to look after 3 under 5's now (whether or not she coped with a number of little one's before may not be relevant)? Maybe she enjoys having your neices/nephews and doesn't want to change the arrangement. Have you asked her?
I think if you want help you are going to have to ask for it, rather than hinting. You would not be unreasonable to ask, but I think you really have to be prepared for her to say no, or to offer for less than you would like in an ideal world.
I think if having 3 children and going back to work was contingent on your parents providing the childcare when you went back to work, then you should have discussed it with them in advance rather than assuming they would be fine with it because they help your sister. It would be great if they would help, but surely you had a plan b?
My parents do quite a lot of childcare for me and have done so since DD was quite small. They do a lot less for my DB's family. This is partly due to distance but also because it was a long-standing arrangement between us (made after ex-H and I split as I work shifts). DM would not rearrange this and cause me significant difficulties with childcare (due to my shift work) in order to help DB (although would obviously help in an emergency). Maybe your mum feels this way too?
I guess what I am trying to say is that your mum will struggle to be "fair" to you both. She may wish to honour a long-standing agreement with your sister, or may not feel able to cope with 3 under 5's. I don't mean to be harsh, and I can see why it stings, but I think your mum is a no-win situation.