All of us who watch a loved one struggle with mental illness are confronted with the brutal suffering it creates, and your question brings to light one of its peculiar cruelties: the disease itself can make the sufferer push treatment out of reach. How should loved ones react? Can you force your brother to take his medication?
The reality is, you can’t. That’s the challenging side of human freedom. Your brother enjoys the same freedom that you do, even though his ability to use it wisely may be constrained by his illness. If he is over age 18, unless he poses a danger to himself or others, he can choose not to take his medication. You could be tempted to react in one of two extreme ways. One extreme might be dedicating all of your energy to worrying about your brother and doing whatever it takes to deal with the consequences of his behavior when he is off his medication; the other extreme might be neglecting him completely or cutting him out of your life.
St. Thomas Aquinas recognized that human virtue lies in the happy medium between two extremes – too much of a good thing actually turns out to be a bad thing. We rely on prudence to help us find the middle ground. You have both a Christian and a fraternal responsibility to your brother, and thus you are called to accompany him in his suffering. But you also have a responsibility to yourself and to other people, and you don’t have to open yourself to needless suffering that his behavior might inflict. Pray and listen to your conscience as you seek to balance these responsibilities. Do encourage him to take his medication, support him in his treatment, let him know that you love him and maintain as much of a relationship with him as is safe for you and other loved ones; don’t let yourself or others be harmed, blame yourself or believe that it’s up to you to “fix” him. Look for the middle ground where you love and support your brother as best as you can but also accept that you cannot do everything for him and that he makes his own choices, even harmful ones, in freedom.