My heart goes out to you and your family, Mebster. What a difficult time for you all!
Bless you and DH for being there for your ill GS, as well as his younger brother. You are clearly making sacrifices to do it, and I'm sorry about that, but trust they will be worth it in the end. Kudos to you for giving a month to taking OGS (older grandson) to that out-of-state rehab, I'm glad DD is staying home w/ YGS (younger grandson), somebody has to.
Not sure how to measure care in percentages. But if you and DH are doing 70% of the care, that means DD is doing 30%, plus perhaps cooking and laundry, etc? It may be difficult for her to face more than that, especially if she needs medication.
In fact, I'm sorry DD went off her meds, and I imagine that might be partly responsible for the drastic weight gain and the fatigue. I agree she should get counseling and resume her meds. But she and SIL have threatened CO if you suggest that again. IDK how they could go through w/ that since they depend on you so much for OGS' (older grandson's) care. But they might have another carer in mind. So IDK if bringing up this topic again is a good idea, unless you're willing to risk estrangement.
What I think you MIGHT do, after you return from the month at OGS' rehab, is to let DD and SIL know you can't do as much any more/have to cut back on the time you give to them. See if the 4 of you can come up w/ a more workable schedule. It may inspire both DD and SIL to step up a little more. Or not. That will be up to them.