Raising my fiances 14yr old cousin.. help please!

I am 22 and have a 3 year old and am currently 15 weeks pregnant with my second child. Approximately 9 months ago I moved in with my fiancé, his grand mother, mother, and 14 year old cousin.. His cousin and I have had a sister type relationship until recently. When she was less than 6 months old, her mother committed suicide and her dad has been incarcerated her whole life, and her grandmother has been raising her. But about two weeks ago her grandmother was involved in an altercation that lead to her shooting and killing a man, leaving me, my fiancé, and her drunk of an aunt to care for her- so pretty much just ME seeing my fiancé is at work all the time. Since I have lived here I have done so much for her such as buy her an iPhone and take her to get her hair/nails done on a daily basis.. Help her with homework, etc.. And just last week we decided that after the Christmas break we would home school her due to bullying over the incident with her grandma so we bought her a brand new $500 HP Laptop.
She has been slacking with helping around the house and has been quite depressed, but given the situation, I allowed her to slack and gather her emotions. I created a chore list that left me with majority of the responsibilities and her just having to do as least as possible.
Well now she is failing school and being a brat and I'm at my wits end. This morning after I told her to ride the bus, as I am sick and didn't want to get out in the cold to bring her to school, she went behind my back after I fell asleep and had my fiancé bring her to school. I'm so upset and lost because I don't know how to raise a teenager?!
I shut off her phone just an hour again in hopes of it changing her attitude but I'm not sure this will work. I'm worried it will just anger her and make her resent me and not change her behavior... What else should I do?! Please help!!!

[AnnieBrooke]

I guess I would re-think the living situation.  Your fiance comes with some very costly baggage no matter what nominal pluses he brings.  You have to take care of his teenage cousin as a price of admission?  Eesh.  Life with just you, the 3-year-old and the upcoming baby is plenty of work, yet probably easier than the load you're toting today.  I'd think about getting out; your fiance can come visit you and the kids.

[Londres70]

Have to agree with AB on this one.  

Who is the LEGAL guardian of this girl?  That NEEDS to be determined by the legal system.  

Where's your fiancé's mother?

I would let your fiancé figure this out and assume stay out of the situation.  

She doesn't NEED iphones, nails done, etc.........she needs a guardian appointed for her and some serious professional counseling ASAP.  This child has been through alot and the remedy is NOT buying her things.