To get the special seating configuration, there are 6 possible places for Trevor (he can't be seated in the seats at the edges). Then, you can choose two positions for one given brother of Trevor, the position of the other brother is then fixed.
The remaining 5 friends can then be seated i the remaining 5 seats in 5! ways. So, the total number of ways you can seat Trevor with his two brothers next to him is:
N = 6*2*5!
The total number of ways of seating 8 persons without any conditions is 8!. If you randomly choose one of these 8! configurations, the probability that it will turn out to be one of the N configurations is, of cuorse, N/8!. The probabiblity is thus:
6*2*5!/8! = 6*2*5!/(8*7*6*5!) = 1/(4*7)= 1/28