I am so sorry about the loss of your dad and your mom's illness. I just lost my mom in June and I was her primary caregiver. My mom had many relatives who lived close and not one came to visit or to help me out. I was home taking care of my 51 year old mom, my 10 year old brother and my 2 year old son. So I can feel your distress in not getting any help. That being said I think it's important that you talk to a Palliative Care if that is avaliable in your area, I know alot of places will provide a sitter to come and just hang out, or do chores around the house or just read to your mom. I didn't find this out until it was too late, well I guess I knew but I didn't think I needed the help, I thought I could do it alone. You can't. You need to be a husband to your wife and you need to be you and not lose yourself in all that is going on, your mom wouldn't want that.
These people who aren't helping out will deal with this in some way, i'm guessing when it's too late, but know that you are doing the right thing and being a wonderful son.
My heart goes out to you, your wife and your mom.