The Family + Friends tab in your contacts allows you to link contacts to each other. To add a new family + friend, click on the Contacts icon and then the contacts tab. From there, select a contact and click on the Family + Friends tab.
Click on new to add an entry. From here, enter the contact's family/friend's name, relationship, and contact information. You can also select to create this person as a new contact as well. This will automatically link the two contacts together.
After you've entered the information and clicked on create new, click on the Family + Friends tab again to see the list of entries to this tab.
In the example above, I added "Example Mother" as a new contact. Then, I navigated to the new contact I created and clicked on the Family + Friends tab, and you can see that the original contact (Ex Ample) is already linked to the new contact I created.
You can also have relationships created automatically by using the family and friends field type in your form builder.