Is the ruler's (brother's) prevention of one from praying in the mosque for political reasons considered a legitimate excuse that nullifies the obligation of congregational prayer?

There is no obedience to a creature in disobedience to the Creator. Therefore, a son should not obey his father if the latter prevents him from attending the congregational prayer and performing it therein. An exception to this is if the prohibition is due to the fear of certain harm or strongly probable harm that would afflict him in his person or property, because fear for one's self or property is among the permissible excuses for not attending the congregation.