..Brendan's trial felt like a complete farce because he was never going to be found not guilty as it had already been established that Avery couldn't have transferred her body from the garage on his own to burn it in his trial, hadn't it..? ...so if Brendon had been found innocent then their conviction against Steven fell apart again...they needed someone/another member of the family to make sure that Steven was convicted and Brendon was the most vulnerable and open to being manipulated, so they chose him...