I am also a care giver to my MIL, although she doesn't have Alzhiemer's diagnosis, she is starting to show tendancies towards it (she is also 81, but is alone FIL passed 6/14/08 during the flood), and contrary to belief, I love her (actually she has been more of a mother than my own). She can no longer bath herself, but does her own cooking, albeit frozen dinners (meals by Elder services), she doesn't do any shopping, but can fold her own laundry if we sort it for her. We do her housework, because it is much to difficult for her to try and use the vaccum or scrubbing the floors. I just wish she would call if she needs something instead of waiting, other DIL shares the duties with me, and sometimes forgets to set up MIL meds, so MIL went without until we just happended to stop by.
My Grandmother had it and from what you are describing, she (your MIL) isn't in the late stages of the disease. My GM got to the point where she reverted back (couldn't comprehend that her daughter, had adult children or grandchildren), which was very hard because she and I were very close all the while I was growing up. It was diffucult when she could no longer remember even her own children.
I applaud you for caring for them. Look for a local support group, for both you and your DH, there are many that assist in helping with the symptoms, the caregiver, and what to watch for. They are very helpful, hopefully you have someone to sahre this work with you so that you get a break it you need one.