My roommates know I RP - I've bullshitted with them about my plans and projects on this very board, or characters I've / I want to make. Given how one is a cartoonist and both of them are nerdy as hell, this isn't really anything odd so much as par for the course, and I've listened to them go on about warhammer aspirations, Diamond league dreams in LoL, and upcoming beta tests. The DOTA 2 championships were like playoffs and superbowl at my house, we invited a bunch of buddies over for wings, beer, and yelling at the TV (Which was hooked up to a laptop, naturally)
Most of those friends would have been coming over to play tabletop games, LAN stuff, drink while streaming movies or play Cards Against Humanity anyway. They're pretty nerdy.
My family knows I'm a nerd, but they don't know about the RP. Mom knows I do a lot of hobby writing, and I've talked about collaborative writing and round robin storytelling with her before. Over t years, she's provided a lot of insights for me in terms of playing women in general. Dad wouldn't get it if I spent hours trying to explain how it works, and wouldn't care if he did understand. He knows I'm a nerd, though. Mom corrects my pronunciation of names from Lord of the Rings and can write in Elvish.
My little brother is a COD-flavored Nerd (more of a 'bro' than anything else, but a benign one) and would likely understand if I talked to him about it, but I haven't.