Be sure the card has the Related Name field: Card > Add Field > Related Name, if necessary, or customize the template in Preferences so the field will appear in every new card. Now, when creating or editing a contact, choose the relationship — mother, father, friend, etc. — from the pop-up, and the person's name in the Related Name field. If the relationship is not included in the pop-up, choose "Custom…" and add it. For example "child" is included but not "daughter". You can add "daughter", if you prefer, using "Custom…".
If you have a contact for the related person, you can click on the relationship entry to open that person's card. For example, if "Adam" has an entry that says "spouse: Eve", click on "spouse" and choose Show "Eve" to open Eve's card.