My brother committed suicide in a mental hospital, do we have the right to sue the hospital and can we win?

You need to start calling lawyers until you have one who agrees to take the case. I am so sorry for your loss. You do whatever you feel is best ignore anyone who judges your question & doesn't offer an answer they are supposed to help you not judge & they don't know what they would do if this happened to them. You actually may help others by doing this. I worked in my local hospital where they kept the person on camera so see if they were supposed to be doing that. The lawyer will be the one who can take care of the details. See if you have any that advertise on TV who specialize in medical cases.