Be able to text your sister or brother, or even your boss, that is by just saying ‘Hey Siri, text my boss’ instead of their name. You have to set up the relationship first by telling Siri e.g. ‘Hey Siri, John Smith is my boss’. And of course that person needs to have been previously set up in your contacts. You will need to say the name exactly how it is in the contacts area when you set up the relationship. The connection is saved on the person’s contact record if you want to adjust it manually.
For Google Now you will need to set it up by doing it the first time, so say ‘Ok Google, call my sister’ and if it doesn’t know who is your sister it will get you to confirm or ask you to choose the contact, and then from that time on you’ll be able to repeat it and Google will call that person.