i dont think she's gay or bi - guys are reading it wrong. It sounds like she built a whole life round this strong person whilst she was alone, someone who because they were a friend, because the relationship was entirely platonic would never hurt her the way a man could. Sadly - somewhere along the line things have got a little warped and they've reverted to playground tactics - dressing alike, going to the toilet together, never being apart. Personally, i thinks it weird - im close to my best mate but there's a line....it stops before the toilet....
Have you tried perhaps booking a long weekend away ...just for you two? or an evening out somewhere..or a day trip? But Maybe saying to her friend 'i really wanted to treat her to something romantic just me and her this weekend, you know her better than anyone - help me out...where would she like to go? What could i do to make it special for her?' and say to your GF the same - that its just the two of you and you're gonna ask for her friends help in choosing somewhere. perhaps if her friend still feels like she has some modicum of control,she will still feel like she was included in the decision making or planning
and your GF can see that you're making an effort to include her friend.....then you can raise the point (very subtley)....that you love it when the two of you are alone - you're never happier blah blah blah.... the key to that is being really open and honest about what you're doing - tell your girlfriend its just you two and make it clear to her friend that thats how its working - but you'd really love her help in making her friend have an awesome time.... then eventually maybe you can start getting some alone time with her.
or, you could just sit down and talk to her straight and tell her you feel like a spare part, that you really like her mate and you're glad shes about but you'd love it if you got some more time together..
Anyway, it sounds like you're a good guy whos got his head screwed on right - each post you've put up you've tried to see it from her perspective or her friends too which i think is awesome, it shows you're not being pig headed or treating the situation badly. Ultimately i think they need to grow up and apart a bit .
"Colour my life with the chaos of trouble"