A friendly, casual chat, face to face, is by far the best way to deal with this. She may genuinely just not have thought about it.
BTW, you don't say how old the 'little brother' is. Are we talking 8 years old or 18? If he's 8, obviously you speak to the lady herself but if the 'little brother' is 18, you could approach him yourself first, and only speak to the lady if that conversation doesn't work.