I agree with the Mamas that said go up there and clean it yourself, or better yet if this is possible, with your husband's family. Depression definitely keeps people from cleaning and doing those daily routines that need to get done, but if you get her all set up, it may be less daunting, and she may be able to maintain things a little better. Plus, the dried feces will be gone, and a little dirt will just boost your child's immunity :)
Other than possible depression, how is her eyesight? Does she go to a doctor regularly? Yes, our senses diminish some as we age, and many people with pets get desensitized to the smells and other pet issues that greatly disgust non-pet owners, but she may need new glasses, or may have undiagnosed cataracts that prevent her from noticing things like dried bits of food on the dishes.
Whatever you end up doing, you are going to run the risk of alienating her, especially if you don't have support from anyone else in the family (support that your MIL can witness - not just verbal support).
Maybe you can just breeze in like a ray of sunshine, tell her how much you love her and that you want to do her Spring Cleaning for her to help her out, that it will be no bother at all, and you refuse to hear any protests. Leave your daughter with a sitter for the cleaning project, but then come back as a family within a few days (at least it will be still clean) and relax and enjoy her company.
Once the house is clean, you could discuss the possibility of having a cleaning service come on a regular basis to help her out. Or, depending on her health and/or age, she may qualify for assistance from something like Meals on Wheels or a home health aid. These are things that will allow her to keep her independence a little while longer, and you will be able to see if she can maintain the home once the initial overwhelming clean is done. Like another mom mentioned, people have different degrees of cleanliness, so it is possible that this is how the home was when your husband grew up, so you may never feel it will be clean enough. Good luck, these situations are always difficult.