Well, my mother was one who believed that wearing a bra to bed would cause breast cancer, so you can be assured that I was never allowed to wear bras to bed as a young girl, never wore them to bed as a young woman...and got breast cancer anyway.
Honestly, if you are concerned, ask her doctor. He could put your mind to rest better than any of us. To my knowledge, it's just an "old wives' tale."
"There's a difference between a philosophy and a bumper sticker." --Charles Schulz