They want half of the value of mine now and his girlfriend has informed me if I don't give them access then the legal fees and fines would eat up my half and I'd be left with nothing.
Ridiculous. Tell them to pound sand. Better yet, since it's a lot of money, find a lawyer and get the lawyer to tell them to pound sand.
His new girlfriend works in law though, and I've received a formal letter from them both
Have your lawyer send a copy of the pound sand letter to the law firm where the girlfriend works, too. She probably sent the letter without their approval unless it's signed by someone other than her. When you look up the law firm, is it a small office, a regional firm, or one of the big law firms? My guess is it's a family shop. Is the girlfriend barred in the state where you live or does she work there in a non-lawyer role?
They want half of the value of mine now and his girlfriend has informed me if I don't give them access then the legal fees and fines would eat up my half and I'd be left with nothing.
Them and what army of unpaid lawyers? Lawyers don't work for free.
I know it sounds selfish
It absolutely does not sound selfish. Your brother made a crappy decision with his money and he's annoyed. That's his fault, not yours. You do not have to share your inheritance with anyone, and should use it for your and your future family's life. It is NOT selfish to set up reasonable boundaries and take care of yourself. You sound young. You need to keep that money invested, go to school, get a job, and not touch that money until you have some more life experience. You don't want the first time that you get taken advantage of in life to cost you half a million dollars or whatever the amount is.
How likely is it they'll win and leave me with nothing? As far as I know there was no paperwork or will just my grandmother's word.
In most cases, your grandmother could have given you everything she owns, given your brother absolutely nothing, and he still wouldn't have a case. It was your grandmother's money to do with as she pleased. She split it equally and your brother wasted his half. It is very unlikely that he'll win.
I have an accountant I've used for years but this doesn't seem like an accounts problem but a law one.
Yup this is more of a lawyer question. You can ask your accountant if they recommend any lawyers. Then, you also need to do research on the lawyer yourself, to make sure they're barred, have no credible complaints against them, etc.
If I were you, I'd get a lawyer to send a strongly worded letter, and that should be the end of it. They MAY still sue - but it's very unlikely they would get far.
Also, stop talking to your brother and his girlfriend. They're scum. I have a similar situation too, except I'm the older sibling. We all inherited a good chunk of change that covered college at a state school, plus some extra to invest. I invested it, and went to work, getting a well paying job, while my siblings are slowly eroding their inheritances. When they run out, that's it - they're not getting anything from me. It's not our job to support unmotivated people. Just keep focusing on you, on getting a good education, and on getting a good career going.