Hi,
I don't envy your situation and I'm sorry you and your family--and your brother--are going through this.
I've worked with literally thousands of addicts as well as family members, and while every situation is unique, family red flags are frequently correct. However, not knowing the details of your brother's history and situation, it would be extremely irresponsible to make an assertion that he is or is not using.
Having said that, whether he is Bipolar I or II, marijuana plays a major factor in medication regulation complications, and many docs will refuse to prescribe until the patient is abstaining from marijuana for a certain period.
Maybe he did do just that, and the meds were able to be adjusted for both manic and depressive cycles (docs have to determine how deep and how frequent those swings are, and which one may be dominant as well--it's complex!). If he did start using marijuana again, either because the meds needed adjusting or because he was feeling too "flat" on the meds, that would throw the meds off and send him into a state of spiraling imbalance.
People with bipolar disorder are often very creative, fascinating, and sensitive people. One of the most difficult challenges, I've heard time and again, is the reduction in that creativity when on meds. For some, this leads to temporarily going off meds to have that feeling again, only to find themselves trying to readjust on their own when things get out of balance.
Being OFF meds for many bipolar people is like being on drugs in a way--extreme swings of euphoria followed by mania (usually accompanied by very poor decision making and impulsiveness that can be life-threatening) followed by a crash, sometimes into life-threatening depression. Their "norm" is a very similar experience to that of a drug addict, and taking meds is like being "clean."
This is all speculation, as your brother's drug or bipolar history and severity are unknown to us, and it's all very complex, actually. I can only encourage you to speak with an addictions specialist one-on-one (not online!).
Many outpatient addiction treatment centers have psychiatrists who specialize medication regulation for people who are "dual-diagnosis" clients in the General Mental Health system--people who have co-occurring challenges of chemical imbalance/mental illness (as defined in DSM-IV) and addiction. Educating yourself about this may help you not only feel you can have an informed dialogue with your brother (for example. if he says he has shakes from not taking his meds, a conversation with a treating psychiatrist may reveal that, in such cases, shaking would not be due to that particular medication, etc.), but may help you and your family make a decision as to whether you feel confident intervening on his behalf.
Now, if you made it to the bottom of this post (congratulations!)...
Hazelden has a good general pdf available on bipolar and addiction:
http://www.hazelden.org/web/public/d...mh_bipolar.pdf
If you want qualified online help from a psychiatrist, you might try here:
Is it bipolar or addiction? - Mental Health
Best of luck to you.