I saw your story, and I'm sorry that I do not have any experience in helping an elderly in-law.
I do have experience in being the caretaker of my mother as her cancer spread. From my experience, the legal documents that give power of attorney, living will, medical power of attorney and declare executor of estate should be handled while she is still legally competent. Those documents won't help you today, but as her demetia progresses ~ those documents will help your husband and his family in handling her legal and financial affairs.
My mother had the insight to schedule a family conference with an estate attorney to discuss all the legalities of how she wanted her affairs handled. I was the designated caretaker with medical/legal power of attorney. Those documents went everywhere we went. Those documents were helpful in getting cooperation from her medical caregivers. Those documents were helpful in handling her insurance, medical bills and bank accounts when the time came and she was no longer able to handle her affairs.
I appreciated her taking care of us, so that we could in turn take care of her.