When I decided to make my charrie a weasley i figured that since the Weasley family is HUGE it wouldn't be a problem. I made it so that Demetria wasn't directly related to Ron and instead made it so that she was Ron's cousins, grandchild.
When I first joined SS, RPing was brand new to me. I've always wanted to be a Weasley, so when I found out this was a Harry Potter RPG I immediatly thought "I finally get to be a Weasley!" After I learned more about RPing I figured it wouldn't be practical if I was Rosie's daughter or something.
Also, when I come across someone who is a Potter or a Weasley I introduce myself a weasley, following with "oh you must be one of my cousins."