[ManakaJunpei]

You should aim to get a lawyer, even if that means selling stuff. If you are separated and she's opening cards in your name, that won't look good. Doing something potentially illegal could worsen your situation further.

I have to agree with this, going into her e-mail might just give her a get out of jail free card and that is the very thing you're trying to avoid
Get a lawyer file a court case and get her disconnected from your life and money, focus on that first then you can try make a good environment for your kids.
Hacking into your ex-fiance's e-mail will NOT help.
Good luck o7