In games where there is a family system it can be a lot of fun for players make family trees linking to each other. Even more so when players can get and wear titles showing their family links to other players. Like "Player 1 Sister of Player 2" and "Player 3 Husband of Player 1" and "Player 4 Father of Player 2". In some games, it is kind of funny to see a group of people all on the same team in town or on a mission as if they all just on a family outing.
In real life, where I grew up it was common to call your closest friends family. With uncle, aunt, and cousin being really close friends. And brother, sister, son, daughter, father, and mother being used as the highest honors. So using those titles in games with people who share a similar culture add that extra level of both fun and realism.
----Not important, just an old player remembering the old days----
Lol, a long time ago (like over 20 years) when I played an MMO for the first time as I was walking around lost a female player invited me to join a guild she was starting. She showed me the basics of the game and give me gear I need for my style of fighting. Because of her help I and another new player soon became well-known players for our high level of skill and being one of the few groups of people who could end game bosses in a group of 2 or 3 when normally most people would have a hard time killing those bosses in 30 and 50 man raid parties. So we made a family with our guild leader as our mother and we as her 2 sons as both joke and sign respect. Even after she stopped playing the game we kept using those titles. We even made up a rumour saying she was a goddess, so after awhile we just became known as the sons of the goddess until stopped playing the game. Lol, she is part of the reason why even now I help other players in games, especially total newbies who may have never played MMOs before.