My sister died this year I know she had a will. My question is who is allowed to see my sister's Will?

Anyone can see the will once it has been presented to the court for probate - it then becomes a public document. So practically speaking there is no reason for the executor of your sister's estate to try to keep it secret.

This should be combined with this - https://www.askmehelpdesk.com/family...ml#post2473668