At some time when it is ‘safe’ walk out wearing a tight white T-shirt that allows ‘things’ to show through. I’ll bet she gets the message.
You could write her a note or and e-mail and tell her or ask her if you could talk and ask her questions like “when did you start wearing a bra” and hopefully she’ll get the message
if you have a tight relationship with youre mother then just tell her
if you and your mum are out shopping go into a store like k-mart or whatever because they have different things there and walk over to the bras section