I can see that eight years ago your DM was eight years younger and she may be seeing/counting on the responsibility for your dsis's kids getting less and less each year as they get older and the before/after school care decreases, and feel she is near the end of this time. The thought of starting again with caring for under fives, particularly three of them, with years more of this responsibility ahead of her, may well be more than she can handle and quite a scary prospect. She may feel that she has done her time.
Yes, I can see why you're unhappy. You would love the opportunities your dsis had, to not have that expense, to be able to put your career first as she has been able to do. Who wouldn't (honestly) feel it's unfair that a sibling had all that while you'll have a struggle? The thing is though that there is no entitlement to this help, and just because she was able to and chose to offer that gift to your dsis eight years ago doesn't mean she is compelled to do equally by you now. There can't be an expectation of her 'doing her bit' for you, she has no responsibilities to your childcare at all and you don't have a right to that time. You also can't expect your dsis takes the financial hit in fairness to help you out- it would be lovely if she offered it, but you can't expect it and don't have a right to it. It's sad for you that you're in the harder circumstances but this is where your choice to have children has to take first place, and if that means for a few years you have to make difficult choices or downsize to manage the costs, then thems the breaks of having children. Other options may be that dh needs to take a career break for a year or two and be a sahd.
The sadder thing is that by doing this lovely, generous thing for your dsis your dm has caused a problem that you feel deprived of that same gift or your turn, and has caused bad feeling to arise between the three of you. I'm sure she didn't see that coming, but that she is sympathising with you but not offering concrete help suggests she doesn't have the time or capacity to give it. If you're currently not working I can see why your dm relies on you more than her full time working daughter, and naturally if you go back to work you're going to have to protect your boundaries. Equally if your dsis is already laying down the expectation that you will be the one to do any caring needed in future for your parents this is where to start clearly stating and managing expectations and boundaries. I don't think YABU for having these feelings, but the hard part is going to be being the grown up about this and reminding yourself your children are no one's responsibility but yours, and you don't have an entitlement. Help is lovely to have but not owed. If there is an inevitable cost to your relationship with your dsis and dm as a result then that's very sad.