If you want to do it manually go into contacts.
Scroll down and select “Related People”. You’ll be brought back to the edit screen and you’ll see a new field “mother” added. If you want to change the relationship, simply tap on the “mother” field and you’re provided with other relationships (e.g. father, parent, friend, partner, etc) to choose from. If you can’t find one that fits, simply tap “Add Custom Label” to define your own relationship.