RP families are really just a for fun thing that friends do basically as fun nicknames for one another.

I was never in one myself, but I used to be in enough RP's (20k posts worth) that I saw a lot of them and got to see somewhat how they worked.
I would not worry about getting into one at all, it's just a fun game people do with their group of friends. It's not any kind of status, rank or anything that makes you higher up or more valuable. 

It's almost the exact same as say two people in spam pretending to marry through a marriage thread. There's nothing actually important you're missing, it's just a thing people do for fun and kicks among people they already know.