OP, you're a saint. You have saved your DD and DSIL tens of thousands of pounds over the years in childcare. Back when my DC were young enough to need childcare, I was shelling out a 4 figure amount each month for the both of them. You've done more than enough already. The agreement you had was for the two older DGC, made a decade back when your situation was quite different.
I can't believe your DD didn't check with you first if it was ok for you to look after the youngest. I actually can't believe she didn't stop and think, "my mum is in her late 70's, Dad isn't well and is needing more care...I am going to talk this through and see how Mum is and whether this is not getting too much for her". I am also shocked that she hasn't made future plans about what to do in exactly this situation: i.e. put aside some of the insane amount of money that she has saved over the past decade to deal with this situation.
My lovely parents helped me with my kids - I was the breadwinner, and working FT shifts, so I understand how bloody hard it is (and how expensive). I was so thankful for their offer, but I also insisted that it shouldn't be full time as I didn't want it to end up being too much for my DP (healthy, active and in their 60's/70's) so my two went to nursery and wraparound care FT. DP very kindly picked up any emergency stuff (or if I had to work nights/weekends/bank hols), and if my shifts meant I needed someone to have the kids between shift start/end time and when nursery/wraparound started/finished, they'd step in. I didn't want them to be tied to an arrangement that was hard work for them, and I wanted them to be able to enjoy their well-deserved retirement. There's a nearly 7yr age gap between my eldest and my youngest (I have 2DC) and when youngest was born, I double checked if they would be ok with doing similar. They were fine with it, but IF it wasn't possible, I would have altered hours or found another job. I also paid for them to have a cleaner and a gardener so that they got some help that way (they didn't want me to pay them at all, but I was able to get about it by doing this to show my appreciation). Every so often, I would check with them to see if they were still ok to continue.
The fact that you've had youngest up to this point has saved them ££££s just in past year alone (let alone the previous 10yrs!). Isn't it once babies are over 1yr old, that fees are slightly cheaper? I'm sure that was the case for when my two were young. I think you need to talk to DD and DSIL now and give them the low down. Don't offer to pay for their childcare!! If you want to continue to have the older two, you're still doing them a massive favour and saving them a fortune (I think it was £25 a day for wraparound care just for my eldest), but do what is right for you. Don't set yourself on fire to keep someone else warm. x