An attacker can easily spoof the name appearing in the from field. This could have either been guessed, found from other information about yourself online (e.g. Social Media profile where your family is listed), or indeed someone has gotten into your Mother's email account and exported the contacts.
I would personally not think it was the latter however, as you'd think an attacker would just use the compromised Mother's email account to send the email rather than using a completely different one.
Probably best to get your Mother to change her email password anyway just in case, and set up a rule that sends all messages from the heiner email address straight to your trash.