OP you are getting some very poor advice from many posters on this thread.
At the end of the day, it's pretty simple.
Forget about the executorship and your poor relationship with your brother. Set that to one side.
If the will said you inherited 50% of the estate, then that is what you are entitled to. End of. I appreciate however that your brother is a jailbird not over fussy about following due legal process. Nonetheless, the law still applies to him, whatever he has done, however many signatures (or wills) he may have forged, monies he may have taken etc etc.
It's clear that, like many people, dealing with this matter does not come naturally to you. Therefore you need to consult a solicitor to pursue this for you, unless you have a business minded friend who can at least undertake some initial enquiries and obtain a copy of the will first. When you go to the solicitor, take with you a clear, concise note of the deceased details, your brother's name, DOB, address etc and say that you want them to find out what has happened to the estate as you believe you are a beneficiary. If you have a copy of the will, take that with you too.
Remember the more of the solicitor's time you take up, the more they will have to charge you, so it is a good idea to be prepared.
Good luck and I hope you get it sorted out.