Q: Why does my phone say a call is from an email address

When my sister calls or texts or face times me it shows up as coming from her email address instead of the phone number associated with her iPhone 4s. When I text her phone number it does not register as iMessage but when I text her email address it does show iMessage. Similarly, I can FaceTime with her via her email address but not her phone number. Lastly, when I factime or text her email address it shows up on both her iPad and iPhone when she is on wifi. What is going on? This seems really really strange to us.