You sister might be bipolar but it could be something else. My dad was diagnosed bipolar and so was I in the summer of 2004. My moods are stable, but probably b/c im on medicine.
If you sister has symptoms like: spending money, talk very fast, stays up all the time (no sleep), overly confident, changes in weight, or is just very impulsive in mood or behavior, sexually active and there are other symptoms but this is off the top of my head....Then she is maybe in a mania.
If she has symptoms like sleeps too much, weight loss/gain, anti social, irritable, no desire to do anything she use to enjoy then she is probably in the depression part of bipolar.
Anyways, its funny b/c im the one who was diagnosed bipolar and my sister and brother are the ones always changing moods...Their happy one minute and then just snap in a second...Anything will set them off. And they are not bipolar ? Or just havent been diagnosed yet? Who knows. All I know is that you can tell if someone is bipolar if they get a blood work up test done...Because it can detect the salt levels in your blood (the litium).
I would just advise you to learn a little more about symptoms online/books before you start getting worried or upset at her.
I know that my sister is my best friend and when I was acting different (i was in my first mania) she just got mad at me all the time, yelled at me and backed off and didnt really talk to me much. And she regrets to this day not being more supportive of me b/c she now knows bipolar is something that is uncontrollable/genetic and something you have no power over stopping.
Sorry----i dont think that im much help. Good luck

I think she sleeps as much as any normal person would,actually less b/c of the hours she works.Her weight has only changed a little since she had her baby so its not that.She does not talk fast. . .I don't know,she just blowes up at people and is mean and rude.Like today for instance, she came up to my job and parked in the front of the building.Security told her she could not park in front of the building and she said "shut up,i can park any where I damn well please!" and drove off.When I asked her to appolagise she sped off.Maybe she just has anger problems? She hits her husband too.She just asts so irate. . .

Wow, yeah I dont know-im sorry. I cant believe she did that to you at your work. Also, sometimes you can't always tell when someone has a mood disorder....I mean those are tons of symptoms I listen and right now, I dont think I show any of those.
Maybe she just does have anger issues? Well, im sorry I was of no help. Good luck.