Ohhhhh dear dear dear what a very silly ex you have :)
Yes, unless his brother is equally idiotic, he's simply trying to upset you.
Hard to know without knowing his exact job but the brother could potentially access personal data. However, he would be an absolute fool to try and do so. Records are kept of log-ins, so if your borther has no reason to access a certain record and does so, or repeatedly accesses one record, then if he is pulled up he is in big trouble. And as for 'fucking your life up' - well, it's a bit hard to see what he could do - copy out your National Insurance number, address, birthdate and hand them to ex? Um, what exactly is he going to do with that information? Sign you up for pilates classes you don't want? If you're thinking that ex-BIL might be stupid enough to e.g. interfere with your records of paying council tax or something - if he did that, he'd end up with a custodial sentence. End of. He won't. He probably wouldn't even dare to access the record of someone with a family connection full stop.
What you do now is report this to the police - ask them what they would suggest you do. Even if they just say 'watch and wait and report anything else to us' - you can reply to ex 'Your threat has been passed to the police. Do not contact me again.' - he'll shit his pants.
Then, you also take this to the council. Ask how you put in an official request that your record is checked for unauthorised access. Name BIL. No doubt all will be fine.
Then you can also say to your Ex 'I have passed your threat onto the council who are going to check access to my record and put a note on my account.' He will DOUBLE SHIT himself and his brother will go absolutely apeshit with him - because it's very likely that this will be the first he's ever heard of this and he will be FUMING that his stupid stalkery brother has dropped him in it with his employers before he's barely started.
Job done.
No way will he try anything like this again.