I too am looking after my mother-in-law in our home. She moved in with us due to failing health four years ago. She was only recently diagnosed with metastatic colorectal cancer.
We have the same problems with family not being involved in her care.
She is a very difficult person but she doesn't have much longer on this earth. You would think either of the other sons would want to mend fences or make an honest effort to make an adult relationship with their mother but it doesn't seem to be the case.
I suggest that you could contact a local community social services group or service and see what adult programs are available. If her spouse was involved in the armed forces, see if she is entitled to some services or monies. That could help offset costs for programs, although they are usually non profit or cost very little. Getting her out is imperative. She needs to talk to others of her peer group and socialize with others. It will give you some "down time" to refresh and recharge. It will give her contact with others not of her family to vent with and encourage her. Look into short term respite care at local long term care facilities, hospices or hospitals. A week can make a whole lot of difference in your emotional and physical health. It is worth it. Who will she have if you get sick from exhaustion?
Do not expect any family member not already involved, to be interested in the daily grind of caregiving. Just wait, they will pop up out of the woodwork when the situation comes near to its end and be there to criticize and make useless suggestions. It's their guilt that will make them so difficult. Keep your faith and resolve strong; you are doing a task that sets you apart from most of society. It is the right thing to do. But be prepared and be honest with yourself if it ever becomes too much for you to take on at home. Caregiving can take so much out of you in a steady, constant manner that you won't know that you are at the end of your rope until its too late. Keep your relationship with your mother-in-law healthy and know when its time to move on to another phase of her care. You and your husband need each other for solace and love; don't lose sight of your future. You aren't alone, we, the caregivers of the infirm and elderly stand behind you. I hear you and feel your frustration and isolation, its endemic to the task.
About all else, take care of you and yours.