Hi, I'd like to help you with your question.
Sometimes when people get older, they have the sense that they can no longer control their own bodies and their own lives. They are facing the ending of their lives and so in order to cope and gain control, they lash out and complain. The pain your mother in law describes may be one of emotional origins rather than physical ones. Otherwise, she would be willing to accept the care offered to her and she would be calmer about her situation, focusing on getting better rather than resisting.
The other possibility is that she has some degradation of her mental abilities and if that has occurred, she may have lost the ability to censure her feelings. She may complain and be difficult because she no longer can cope with her own emotions.
In any case, the situation is causing you and your husband distress. First, know that you are not alone. This type of situation is very common when children deal with elderly parents. Parents are often uncooperative and emotional. Two, realize that you both are doing the best you can. You have done a lot for your mother in law, more than many people consider doing for their parents. You have gone above and beyond. Feel good about that. Your mother in law is not alone and she is well cared for.
One of the best ways to help yourself and your husband is to educate yourself about caregiver stress. Caregiver stress, or caregiver burnout, is an increasingly common condition. Knowing the symptoms and the effects is very important so you can realize when you need to stop and take a break. Also, support is very important. Anything from attending support groups in person or online to contacting local agencies to help assist in your mother in law's care is available. Knowing that you are not alone and sharing resources and answers can help lift the burden off you.
Local agencies such as the Area Agency on Aging can help you with services such as companions to stay with your mother in law and check on her, Meals on Wheels, ACCESS transportation services and other resources can help take some of the burden off of you and your husband. Contact your local United Way or city/county government for contact information.
Here are some other resources that may help you:
How to Care for Aging Parents by XXXXX XXXXX and Robert M. Butler
Coping With Your Difficult Older Parent : A Guide for Stressed-Out Children by Grace Lebow, Barbara Kane and Irwin Lebow
The Complete Eldercare Planner, Revised and Updated Edition: Where to Start, Which Questions to Ask, and How to Find Help by Joy Loverde
You can find these books on Amazon.com or your local library may have them for you.
Let me know if I can help any further,