Hi,

You could use one of these scenarios:

1.
You create a custom table where would be the ID of user: father, the second user ID and ID of relationship type (e.g. son).

You could use the custom tables module

This approach allows M:N relationships.

2.
You would create a custom user field (system tables overview) . The field would store e.g.:
John/son
John/wife
John/daughter

This approach allows only 1:N relationships.

If you need find all related users for John you search for : LIKE ‘John/%’. Or you may use the user id for cases when John changes his name.

In both cases you need to develop web parts for adding/creating/editing users by yourself.

Best regards,
Juraj Ondrus