It is always difficult to be a primary caregiver, more so for a relative.
Do not expect help from family members who have not already offered assistance that is actual and real. Some people cannot handle the emotional and physical demands that caregiving entails. It is not your fault that others can't see their obligations and responsibility. Their sense of guilt for not being there for you and your mother is what makes them either offensive or useless. You are doing the RIGHT thing in caring for your mother, never for a minute doubt that.
Having said that, it is very important that you take care of you and your wife and family, emotionally and health wise. What would happen if either of you became seriously ill due to stress? Stress is a cause of illness and death so don't disregard your levels of tension.
Look to local social services to get ideas for short term respite care in hospitals, nursing homes or hospices. A week to recharge can make the difference in how you look at your mother, her illness and your life. It will also be good for your mother to socialize and for a change of environment.
Have your family physician do a stress test on you and your spouse. Armed with a diagnosis of marked stress, it will make it easier to get the attention of social services. Using social services isn't giving up, it just is using the help that is available. Does your employer offer sympathetic care time off? Was your dad in the armed forces? Maybe your mother qualifies for some help, either services or monies to offset costs of services. There are many companies who don't charge extreme fees, i.e. HomeInstead that can send a careworker for a couple of hours ($10-20 an hours) to your home and take your mom out shopping or for a stroll in the park in her w/c. It would do you all some good. Many agencies have volunteers that can come and sit with your mom for short periods. There are adult day programs run by medical personnel that are reasonable and enjoyed by the clients.
It is easy to get overwhelmed by the responsibilies and worry of caregiving. I feel isolated and helpless sometimes in the face of the enormity of the task, but it is the moral and ethical thing to do, whatever your belief. I take some comfort in that. We do society a service but best of all we stand up for the giving back to our family in the circle of living.
We stand with you, we honor your committment and feel your doubts, but we stand strong together.
Take care.