Oh, I feel your pain! It can be so hard to talk to relatives about money. Even more frustrating when you want to help, but they only want to talk about how poor or overtaxes or somehow oppressed they are but don't want to actually do anything about it.
Someday when you are feeling especially calm, you might try saying something along the lines of, "You know Mom, it makes me stressed out every year when I hear that you are having tax problems. You sound stressed out, too. I would like to help so that we can both be happier."
Either she can take it or leave it. Hopefully, if she won't take your help, she will at least stop talking to you about it. Good luck!
It sounds like they just don't have enough withheld (if any) and they don't have any savings, so when it's time to do taxes even a couple hundred dollars would be a big deal to them. Plus, some people just like to complain about taxes, like it's a requirement to hate them. I know someone who starts complaining in Dec.
And continues until April and last year I helped him file and it took 1/2 hour and it was so easy. He never stops complaining though.