Wow. After reading all of the updates (btw, what a loving and respectable relationship you guys have /sarcasm) you guys are the ones who need the help. 

FIRST AND FOREMOST GF/Spouse/Anti-Stepmom she is 16 years old and she is entitled to be awkward to be around, you selfish shit. Maybe try connecting with her a little more because even I don't understand your family dynamic. I find the weirdest thing about your "un-blended" scenario is that you think that a 16 year old isn't going to be awkward around people she can definitely tell don't want her around. You can't be in a relationship with someone who has kids, much less kids that are teenagers, and just expect them to avoid you or you avoid being around them. You knew what you were getting into when you got into a relationship. Instead of getting to know a 16 year old you thought you would take the easy way out and just separate things and make it weirder for anyone that comes into contact with you on his families side. Way to go girlfriend/stepmom/whatever. What a coward, I mean honestly. 

Now onto you, Dad. You should definitely grow a pair. Seems like you haven't cared about the scenario for 3 and 1/2 years and that's a problem. Your kids will always be in your life and they are entitled to be around the people you date or are married to, because surprise, that's where you happen to be. It is weird to think you can just avoid mixing the two, especially with a kid still so young. You honestly must be colorblind to all those red flags around your joint/un-blended home because that should never have been an arrangement.... ever. Also, maybe you should reinforce better behavior with your daughter by spending more time with her. Then again, I don't know if it will help that unforgivably awkward 16 year old out. Usually kids get no personality or behavioral traits from their parents so who the hell knows where she got that from. The last two sentences were sarcasm in case you can't pick up those flags either, you douche bag.

Maybe I've just become biased after reading this cancerous post, but I think you guys are toxic. Get away from each other or go to counseling. Seeing as you guys might be blaming your problems on a 16 year old, she should DEFINITELY be the one getting counseling just for having to be around you guys.