All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
At first, we pray that Allah blesses your brother with a speedy recovery. In case trustworthy doctors decided that giving him medications will only cause more harm and produce more severe disease symptoms, and that the best course of action is to stop the medications, then there is no sin in that. However, this is provided that the decision is made by a committee of trustworthy medical doctors, and neither the doctors nor the patient`s family are considered sinful since they have done their best to save the patient, but the medications didn`t work. This is attested to in the following verse: "On no soul doth God Place a burden greater than it can bear." {Al-Baqarah, 286}. And Allah the Almighty Knows Best.