If proper modeling of behavior and adequate consequences have always been in place AND if this child stands out as drastically different behavior-wise from her siblings AND all methods of behavioral intervention/modification (designed by medical professionals) have been exhausted AND I would also require all possible lab tests to be done rule out any potential medical cause (lead poisoning, hormonal imbalance, etc).

If after all of that (which could take 6 months or longer) the issues have not resolved or I have not even seen the slightest glimmer of hope, then I would research medications available.  I would consult with multiple professionals about the side effects and benefits of the medications recommended.  I'd have to be absolutely certain I explored every other avenue before using psychotropics on a child.

With all that said there are clear cut cases of bipolar AND schizophrenia in childhood, though they are RARE.  I would think if biploar were the suspected cause, I would get a second and third opinion to rule out ADHD (hyperactive or combined).  If there were disagreements between the professionals regarding ADHD vs. Bipolar, I would try to treat for ADHD first.  ADHD is much more common AND has medications that have been better studied for use in children.  But I'd only do so if the professionals can't decide and there is no family history of either.  If the ADHD treatments do not work, then I really would consider it possible to be bipolar and would explore those options.

Any way you look at it, there lies a tough road ahead of you.  There may be unexpected twists and turns along the way too.  Whatever the decision, seek out support in your community, if available and if not, seek out an online community with other parents going through this.  There can be a fountain of information coming from other parents that have been down this road before you. 

Best of luck.  HTH