Op, being soft hearted does not mean you should be a doormat, and to be honest, it sounds as if both you and your parents are taking your husband, ( and your sisters) massively for granted. How disrespectful.
My DH was you , for his mum, with siblings that had a myriad of excuses. When he could not help, things fell to me, and I resented that hugely. So I had to learn to say no.
I just stepped back and was not available , other than for emergencies / things which could not be pre-planned.
MIL figured out how taxis worked; prescriptions were delivered, as was food. She had the assessments she needed , and actually listened.
Step away ; invent an illness, or have your DH have one such that he cannot drive them, if that’s what it takes.
Continue with empathy and compassion, but don’t be the family drudge.