I'm not sure its helpful to tell the OP to call an ambulance now. The OP has been crying out for her DP and his sister, who are the MIL's children, to step up, make a decision, care for their mother in their mother's home nearby, or the SIL's home, or call an ambulance, or arrange a hospice bed, or clean up after their mother, or support the OP who is dealing with this and her own disability and her children with disabilities.
To the point that she is telling her DP, father of her children, that if he doesn't take action she is permanently leaving because she doesn't want her MIL dying in her house/bed and/or her children witnessing their grandmother die.
From where I'm sitting, the OP is being very reasonable but no-one, not family, not GP, not district nursing, not cancer care, not hospice care, not palliative care, not the hospital, no-one, is taking any responsibility, except for the OP.
And she is watching her MIL die and she cares very much for her MIL, but is at the end of her tether. This thread is an outlet for her, and we need to support her, not berate her at this very distressing time.
I'm not surprised she's telling her DP that she's leaving forever with the kids. Because in these circumstances I'm helping her pack and get out of the door.
I'm so sorry that no-one is taking responsibility for your MIL, OP. Now is not the time to make decisions about your relationship, but later on, when you've had time to reflect, remember what blood relatives put on you. Remember that. Because they've shown you who you are.
Sending hugs 💐💐💐