So I wasn't aware if people already knew that you can give contacts any nickname you want on Siri. It's really easy to do, just go to contacts, select the contact you want to give the nickname to, edit, add field, nickname and you're done. From now on, you can refer to this person as either their nickname or their actual name and Siri will register it as that contact, exactly like how "mother" "brother" etc work, except it can be any word you want. (I call my girlfriend Beans, for example.) Trying to give people nicknames within Siri (telling it X is my Y) limits it to family names and relationships, and won't allow you to give personalized names.