My mom can't send me iMessages and she has iPhone4 and I know iMessage works on my iPhone 5
I'm with you, this pisses me off. This small explanation may help:

iMessages on an iPhone has two ways of sending a text.

iMessage


  This is the blue colored bubbles. It requires data to be turned on to
  send and receive messages. It goes through apple servers. This can be
  sent between macs, iPads, iPods, and iPhones.


SMS


  This is the green colored bubbles. It does not require data and it
  goes through cell towers from your carrier. This can only be sent to
  phones, including android phones, those old Nokia phones, and anything
  that can receive text messages. It does not send to a mac, iPod, and iPad without cellular


On an iPhone you can have both enabled, or just SMS. You cannot disable SMS.