You need professional advice from a solicitor, if your Gran had a solicitor who is not the same one your Uncle uses ask to see someone. Ask them if they ever had a copy of her will, your Uncle may already have acquired this if he knew about it.
Like Stokeace says your Uncle sounds to be devious, this is why you must see a Solicitor your Uncle does not use.
I may be wrong, so ask your Solicitor how the money should have been shared if there is no will, as I believe that the money should have been divided between your Grans children, not just the surviving children, and you and your brother should have received your late mothers share.
To start with only ask for advice from your Solicitor , if he starts requesting wills or writing to your Uncle it will cost you a lot of money.
When know what should have happened you can contact your Uncle yourself, your Uncle may back down when he realizes you know what you are talking about, if he still refuses then ask for a Solicitors letter to be sent.
If you find that there was a will which has been through probate you can obtain a copy from the Probate office in your area at a cost of about �10.
Don't forget to ask for interest on any money you find you are entitled to, there are times when you are not entitled but it is worth a try Good Luck