In very relaxed contexts in which you can joke freely with your friends, you may say it, pretending to be cool.
Or on the Internet, in forums, online chatting and so on.
妹只是个传说 is extremely rare. In today's culture, 妹 stands for "cute girls", which doesn't fit too much the cool image of 传说中的哥...
The counterpart of 哥 is 姐, "elder sister", who sounds as strong as 哥. It makes more sense if it's 姐只是个传说.
Your big bro me is just a legend, so don't just worship me. See here I use "just" twice in English to express some subtle complex originally in Chinese. I say don't worship me, but I mean the other way round.