G.T.
I also have a close bipolar family member. It took several years to get him on the correct dose of medication to eliminate the violent mood swings. She shouldnt be acting out like that if her meds are right. If you have spoiled her you need to undo that. What techniques do her dads use? You might need to take on some of their dicipline methods. Don't let her run the home, it will only get worse as she gets older and learns how to "play" her disability in her favor.
There are bipolar chat boards, it might behoove you to read and post at some of them for some answers and suggestions. You and dad shouldnt be walking on eggshells, she needs much more stability, routine, and boundaries set for her because her condition does not allow her to foresee consequence on her own. Place them, and discipline her immediately when she over steps.... if she's on proper meds you need to not treat her like she's different, it will blow up in your face.