Mercenaries usually want to live and spend their money, not don a suicide vest.
If they were paid operatives, it seems like they would have had enough money to get the heck out of Dodge. :)
Seriously, I think the older brother was a loser, got involved in Islamist jihad as a way to find some kind of purpose in his life (eyes rolling backwards), and then involved his younger brother.
Sleeper cells...part of a big group and sometimes the actual bomber doesn’t know the full operation or the plans they just do it.