I too am very sorry to hear of you mother's illness. Cancer is a horrible disease. I lost my father to Leukemia last year and have lost too many friends, co-workers and family to the disease in the past. One can only have hope that someday they will have a cure. I know it probably isn't any consolation but a co-worker of mine had the same form of brain cancer and was initially given 6-12 months to live. He passed away almost three years after his diagnosis and his quality of life was relatively good until the last couple of months.
That being said, you already stated in your post that you are dealing with an insurance company and not the airline in this matter. Even though the insurance company has denied the claim you can still appeal to the airline which, from reading your post, it appears you have already done. Hopefully AA may have some compassion and work something out with you. Perhaps, for example, instead of a refund if it's not offered you can ask that the tickets be transferred so that you can visit your mother.
In addition to this all insurance companies are regulated by each state in which they do business. If the clause about pre-existing conditions wasn't clear at the time you purchased the coverage, I would suggest that you contact the
Texas Department of Insurance. They may, however, refer you to the state in which your mother resides since the insurance was purchased for her.
Finally you should contact the issuer of the credit card used to purchase the tickets. Often there is some form of automatic travel insurance but the scope and restrictions vary from card to card. They, too, may have an "escape clause" where a pre-existing condition is involved.
Good luck.