First, let me just say that you've been through a rotten 'welcome to polyamory' experience! It just sounds as though your gf and her bf didn't get their act together before-hand and now you're suffering the consequences.
Before getting into each part of your story, I just have to ask, didn't your gf sit down with you and talk to about what you wanted/expected from the relationship? Didn't you talk to her about what she expected, and what her bf wanted? It just reads as though you all fell into this mess and there wasn't much communicating beforehand.
So if there wasn't a big sit down, where everyone had a chance to explain exactly what they wanted and expected out of the relationships (yes, there are multiple relationships here, yours and hers, her and her bf, her bf and you, the 3 of you together) then that's probably the first thing to do. Without that very important step, almost nothing else can be resolved. Open, and very honest communication needs to be the bedrock of all the relationships among the 3 of you.
If they aren't doing this (and they should), then you need to demand it. They are playing with emotions and if you don't watch out, you could be the one getting hurt. So ask for a meeting, tell them they need to be prepared to say exactly what's on their minds, what they want, and what they expect from you and the relationship.
Good luck and I hope that everything turns out good for everyone involved.
Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way. - C. Hitchens
Me: Male, bi, 48, flexible