How can I help my sister?

[Hurtingall_over_and_sick_of_this]

So, my twelve year old sister is having problems with what seems to be possible bipolar disorder. Before suggesting to my mother to bring it up and see a doctor, I want to know if she sounds like she may be bipolar.
First off, she is constantly shouting at my brother for "bothering" her. No matter what he does, she gets upset out of no where. Sometimes she'll be fine and overly happy while hanging around him, but every do often she'll flip.
Also, she tells really reeeeaally long stories that take FOREVER! I'm not sure if this has anything to do with anything, but it's worth bringing up. Sometimes she won't respond no matter how many times I call get name, sometimes she acts like a four year old, sometimes she is sad about the smallest things. It could be simply depression in a different form than my own, but I wanted to be sure of possibilities before sending her to a doctor.
Thanks in advance!

[Debdoc93]

Bipolar is not an easy illness to diagnose is teens, but her behavior does remind me if my daughter.  (She was 17 when diagnosed) Only a dr, after an evaluation and unfortunately some time, will know.  The biggest concerns are the depression (if she's self harming or has suicidal thoughts) and the impulsive behavior that comes with mania or hypomania.  My daughter had rapid cycling which ment she would go from super high energy and happy to angry to extremely sad all in the same day.  This is not uncommon for teens with a mood disorder.  I would get her some help. A good dr who really listens.  Just some FYI, antidepressants, without a mood stabilizing medication, can  make things worse IF she actually has bipolar.  We found that out the hard way.  Best of luck to you!