Bipolar disorder aka Manic Depression is a difficult illness for many who have it. It is even
more difficult on the family. Family is often the ones who have to pick up the pieces...sort of like damage control after a raging tornado set loose in the house.
Bipolar Type I appears to be what your sister suffers from (as described by the picture you paint)...with the high manias alternating with the low depressions. She also may be suffering from what is called "rapid cycling" which occurs about 5-15% of folks with the illness (4 or more mood episodes during the past 12 months)....which has a poorer prognosis.
Approximately 10-15% of adolescents with recurrent major depression will go on to develop Bipolar I. Bipolar I is a recurrent disorder with 60-70% of all Manias developing right before or after a Major Depressive episode. 60-70% of folks with Bipolar return to a fully functional level between episodes...which is good news. 20-30% do not and continue to suffer impairment even when out of the throes of it. Many folks, especially when they are young, are totally non-compliant to treatment because of denial or stigma. Think of the adolescent or young adult who is newly diagnosed with diabetes...and refuses to take medication and eats all the chocolate and sweets any way. Many folks unfortunately have to hit their rock bottom before reality really sinks in and that they cannot do this any more. Despite their dislike for meds and the illness, they finally come to realize that Mania is not what it is cracked up to be. Many become noncompliant because it "slows" them down. Yes....that is the intent. In mania, they lack brakes. Medication gives that person a pair of brakes...actually, this empowering, not disempowering.
Not all folks with Bipolar I, like any illness, have the exact symptomatic picture...there is some variation. Some folks who are Manic are happy go lucky and are very likable as they ride their manic high, some become very irritable (even mean). But typically, the more the mania, the more the excess energy, the more the impairment. In mania, the person tends to run where the impulse carries him/her...therefore, more impulsivity. This is where individuality of the illness becomes apparent. Some folks become impulse spenders, some folks become impulse travelors, and some become impulsively promiscuous. Many begin multiple projects out of inflated self worth that later become left unattended to later on when the manic high runs out of steam. Many come to enjoy their manias because it feeds more energy into their creative/artistic endeavors. Where the impulse takes them, they tend to go....without much thought regarding consequences...and many may often become irritable when prevented from carrying out their impulses. Self medicating via substance abuse is very common...another impulse...because it feels good or it is a poor attempt to self regulate their illness in their own way. However, many folks in the throes of mania, despite the havoc it causes, tend to also look upon their manias quite fondly....reluctant to let them go...because they have all this energy...doing and experiencing things very unlike their depressions (which leave them little energy). Due to impaired insight, they often fail to see that the very mania that they come to love and enjoy actually causes more stress for them, which just feeds into their stress load....like a squirrel in a wheel...unable or unwilling to jump out of it. Like any illness to any extreme, the higher the mania places them at great risk for psychotic episodes as well, much in the form of delusions. Delusions are fixed, false beliefs...and some of them can become quite grandiose and even bizarre. Paranoia is not uncommon. When a person is highly manic, delusional, irritable and paranoid...it is not the time to confront them...but to call in the authorities to bring him/her in for involuntary treatment....especially if family believes that the person is at risk to harm self or another. The family member may need probated by the courts against his/her will then. As you can tell, the higher the mania, the more the impairment in insight and judgement in order to make rational decisions, especially when things are done at the impulse. The illness is hell for the person who has it, but also doubly so for the family who have to undo the damage and pick up the pieces...because the person is unable or unwilling to do so. Burning out the family is not uncommon either.
Family really needs to get all the support they need in order to understand the illness for what it is, how it impacts that family member and them, how to stay on top of it, and learning what they can (and cannot) do when mania raises its ugly head. NAMI (a family support group) is a nation wide support system devoted to families who have a loved one with a significant mental/emotional disorder (like Bipolar and Schizophrenia). It is there to support, educate, and help offer suggestions for family. Most families who use the NAMI groups find them extremely helpful...other folks who have been there and understand. The NAMI website is
http://www.nami.org/ From there, you can locate a local chapter to assist you.
There are many books out there on Bipolar disorder. Do a web search or visit any library or bookstore. For family, knowledge is power. Coming to understand the illness makes it less scarey for you and empowers you as family members as well. One book just to start you off is:

Which can be found here:
http://about.pricegrabber.com/search...=about_bipolar&
Bipolar disorder is hard disorder for those who suffer from it. Many who have it, despite the manias, really do hate the disruption it causes in their lives. Many folks who have it do, however, lead better productive lives when it is managed well...which means medication. Medication is key to success. Without medication, failure. There really is no compromise. It is a medical illness that requires medication. Since there is a significant genetic loading for this illness, I would tend to agree that your father most probably suffers from Bipolar disorder, or maybe another mood disorder. This also places you at some risk too...so the more information you come to know about the illness for your sister, it may also become helpful for you too...if you should develop a pattern of depression, placing you at risk for mania since it is present in the family already. Knowledge is power and is empowering to you.
In knowing about stress and stressors, it is correct that the more the stress, the higher the risk in developing depression. So, learning how to manage daily stress is key. Learning better coping skills, balancing work and play, having friends and supports, giving oneself little time outs to decompress, and improving one's diet and sleep. All key to reducing one's life stress. School can be very stressful, so this is doubly important here.
I hope this has been a little helpful to you. I encourage you to learn more about the illness, to take care of yourself, to get support (like NAMI), and to continue on with your life despite the mania/bipolar in the family.
You have my very best,
Wolfie