So, it's a quite complicated story, but I'll keep it to the main points... I met this 4 set a few weeks ago and you could say I befriended all of them... I got the number from all 4 and gamed my target via text (didn't really have much chance before).
I met again with her and her friends two times and the second night out she practically jumped at me to make out after I isolated her. But she told me one of her friends can't find out because that friend would kill her (a fattie who's into me). A few day later I f-closed her and now the problem... I hang out with that group sometimes but that particular friend is almost always around and sleeps over at the HB's place 2-4 nights/week (like I said quite complicated). And the HB still wants to keep everything a secret from the fattie (but all other friends know).
So what can/should I do now? I mean I already told here it's kindergarten but she just said I like to play kindergarten... I thought I would just stop calling/texting her and wait for her to come and then tell her I don't like to play games, etc. (which could be a bit complicated because I'm also friends/in contact with her sister and another friend of hers, but still...)

Tell her to stop playing games or you're done with her. There's always another girl out there and dealing with BS drama like this isn't worth it.