It's no more "harmful" than wearing a properly-fitted corset the same amount of time.
Sure, if you have a 30" waist and try to wear a 19" corset with no training, you're going to hurt and cause problems for yourself - just the same as you can bind badly or incorrectly.
Cancer is cells growing where they're not supposed to. I don't see any medical reason why light to moderate pressure would cause this, but I'm not an expert.
I wouldn't use ACE at ALL. It's just dumb, and you can buy wide elastic bands for cheap. You CAN hurt yourself using ACE, even for under an hour of wear.
Honestly, I would guess that your mother has other issues with the idea of you dressing as a boy - not any actual issue with health worries.