The issue you have when trying to use Siri to make a call by saying, "Hey Siri, call my mother" is that unless Siri knows who your mother is, how can 'she' call her?
To get around these types of issues you need to populate your Contacts app with the information (metadata) it needs for Siri to action those sorts of requests.
Below are a few examples of what I mean:
- If your mum's name is Charlene and that's how it's entered into your contacts, then saying, "Hey Siri, call Charlene" would work. Likewise, if you change her Contact details to Mum instead of Charlene, saying "Hey Siri, call Mum" should work. All of this is because Siri now knows who you're referring to.
- If you have your mum's birthday in her Contact details, then saying "Hey Siri, when is Mum's birthday" would work. Of course, if you have no birth date listed, Siri has no way of answering this question.
- If you don't have a 'home' address entered in your contact details, then saying "Hey Siri, direct me home" isn't going to work.
- If you don't have an 'email' address entered in your Dad's contact details, then saying "Hey Siri, email Dad about dinner" isn't going to work.
In all of the above examples, the only way Siri can action a request accurately is by knowing the necessary information. And the only way Siri can know that information is if you've provided it somewhere to begin with, such as in your Contacts app.
If you specifically want to be able to say the words, "Hey Siri, call my mother" then you'd have to do something like change her contact details so her name was listed as "My Mother".