As I understand it, the central issue in Borderline is abandonment. If there is explosive anger, it is provoked by the threat of being left alone (i.e. abandoned). If there are compulsive behaviors like eating, shopping, drinking/drugs those arise from the need to avoid feeling alone at any cost. Often these behaviors are reinforced because they lead to association with other people (think the bar or the mall) who fill the person's need for companionship while keeping the relationships shallow at best. It is the illusion of friendship that the person gets and that is all that the person can sustain. They will interpret these relationships as deeply meaningful despite a great deal of evidence to the contrary.
One of the hallmarks of Borderline (BPD) is suicide attempts. One can view compulsive behaviors like eating or shopping to excess as potentially self-injurious and in the realm of suicidal behaviors. These behaviors are usually impulsive. The person may be hyperfocused but it is not well sustained.
This is because the person with Borderline (BPD) is highly reactive to their environment. The smallest thing - real or imagined - can set them into a frenzy of ego defense. Their emotions are not so much swinging or shifting (as in Bipolar) as in perpetual motion between extremes. The person with Borderline (BPD) is both afraid of being abandoned and afraid of being engulfed and they are unable to inhabit the territory between these extremes.
I have this image of an atom of sorts. The center where the nucleus should be is a black empty space. The electrons are moving around the shell of the atom chaotically because there is no central organizing force (the nucleus). The electrons bounce wildly around, always avoiding the center. The atom cannot join with others more than briefly because the electrons cannot truly bond with another atom. At any given moment the electrons are attracting other atoms only to repulse the other atom when the joining threatens to bring stability and an organizing force to the molecule they will become when joined.
This chaos of feeling is usually expressed very clearly in the life and behavior of a person suffering from Borderline (BPD), at least for the first several years after onset. It is universally devastating to their intimate and familial relationships. They are often unable to hold a job for very long. They may move frequently or get entangled in dramatic squabbles with neighbors or landlords.
The good news about Borderline (BPD) is that it is treatable with DBT and that many people recover from the more severe symptoms within 5-7 years of onset. Borderline (BPD) features are likely to continue to plague the person but may not be nearly as dramatic or disruptive over time.
The latest research on treating Borderline (BPD) suggests that both mood stabilizers and AAP's can improve global functioning. AD's may be helpful with depressive symptoms but don't seem to be very effective in reducing anger and impulsivity.
So... your description of sister in law doesn't seem to fit really well with Borderline (BPD) but it certainly could. Borderline (BPD) has been reported as comorbid with depression in up to 70% of patients. Other comorbidities cited include eating disorders, Bipolar, PTSD, Nacissitic and AntiSocial personality disorders.
She could be alphabet soup.
Over the years my experiences and discussions with therapists and doctors has left me with the realization that the label really doesn't matter a whole lot. For one thing it is very subjective usually. One person's Bipolar is another person's Borderline (BPD). Focusing on the symptoms/behaviors is much more helpful. As I am sure you know, having a hx of a mood disorder of any kind is associated with increased risk of developing a mood disorder - of any kind. Trying to back into a diagnosis based on what medications are effective can be helpful but not conclusive evidence. Since Wellbutrin is a multiple reuptake inhibitor that we know blocks Dopamine and Norepinephrine and Prozac blocks Serotonin, her brain is stewing in all three neurotransmitters (compared to pre-medication). Since we're talking 3 of the major neurotransmitters here, it's a little hard to say what diagnosis to slap on. I agree that bipolar seems unlikely since there is no mood stabilizer on board and she apparently isn't manic on the AD's.
Therefore, I would put my focus on discerning symptoms that may have a heritable basis and that may require different treatment approaches:
1) is there a mood disorder present?
2) is severe anxiety present? phobias, panic attacks
3) is psychosis present?
4) is there severe impulsivity?
5) are there clues suggesting obsessions and compulsions?
I would label the mental health family tree based on these kinds of constructs rather than "diagnoses" per se. You could do a spreadsheet with the symptoms of interest (rage, mood swings, obsessions - whatever) and list the family members then just check each area that you have evidence for that symptom. Might show you a clear pattern but probably won't. It may make you more aware of a trend or the absence of a group of symptoms.
Have fun - you know you will!