Whatever the relationship between stress and cancer, my opinion is that there is little if any likelihood that you are the reason for her cancer. Especially in only one year (2007-2008). Such thinking is part and parcel of the confused reasoning of depression. If you would just do the math, you would realize that if worrying about your child caused cancer, virtually every parent would have cancer.
It does not sound to me like you are a parent: I am and have been for over twenty years and trust me when I say that I have worried, worried, worried about my child -but that did not cause my cancer. HPV 16 & my immune system did.
Focus on comforting your mother and being a good caregiver. You are not helping her or yourself with this negative fantasy.
Charm