tough situation mate. But i have been in it up to my ears and at the end the truth usual comes out.
I was keen on one of my sisters friends (similar situation, kinda) and we hooked up etc and went out for a while. The problem is however my sis lost her friendship, like you probably will, because the only time my sis saw my gf was when gf came over to see me. In the end their closeness was ruined because they never got to see each other and ultimately my sis never saw me, then i guess jealousy at both of us set in.
What im trying to say is that it's a difficult situation which you'll have to way up the pros and cons but all you can see now are the pros...
- Imagine going over to see your mate....she will be there should/will expect preference of your attention (she's a chick it's natural
- if you break up you lose ya best mate and your gf
My advice: If you are really keen, I mean like L word keen, then i reckon you should tell the girl first of how you feel, then tell ya mate if she feels the same. see how they both take it then if it doesnt work out, then no one was ever in the dark and you can still be cool with the bird
AND REMEMBER MATES ALWAYS COME FIRST....IF YOU DONT LISTEN TO THIS ADVICE THEN YOU WILL ALWAYS, ALWAYS LOSE AND YOU'LL FIND THIS OUT THE HARD WAY.
Best of luck and keep us informed