No, not really - as you yourself pointed out earlier, the FBI has lots of people on their books as suspected terrorists - not all of them actually are terrorists in practice, clearly, or we would be inundated with terrorist attacks non-stop. I have yet to see any evidence that implies the younger brother was known to be a terrorist - and he is the one due to go on trial.
Personally, I find the drip-feed of incriminating 'evidence' that is then later rescinded and turns out all to have been a lie eg they robbed a 7-11 at gunpoint - oh no they didn't; the younger brother was armed and shooting at police - oh no he wasn't etc - gives me diminishing confidence that any other part of the story holds water either.
So far, the only 'evidence' we have been presented with that matters, in relating to the suspect, is that he was videoed putting down the bomb and it then blew up - if this is truly the case, then clearly he is guilty. But so far, we have only assertions by anonymous sources that this video evidence actually exists. If it is as reliable as all the other 'evidence' of guilt so far, ie as illusory as the guns the younger brother was supposed to have and be using, then I am not very convinced, no.
Even if it can be proven that his older brother was guilty - which is far from proven (being a nasty piece of work may be coincidental rather than causation), no-one should be judged on the sins of their brother.