Once probate has been granted a will becomes a public document which anyone can obtain a copy of. By entering a 'standing search' with the Probate Registry you'll automatically get a copy of the grant of probate (which includes a copy of the will) as soon as that grant is issued:
http://www.justice.gov.uk/courts/probate/standing-searches
If there is no will, the rules of intestacy will apply. See here:
https://www.gov.uk/inherits-someone-dies-without-will
Note that, unless your mother's estate was very small (and her affairs uncomplicated) it will be necessary for someone to apply for 'letters of administration' to enable them to distribute the estate without a will. You and your sister (and any other siblings) have
equal rights when it comes to applying for letters of administration. (i.e. either/any of you can make the application, either solely or jointly). Such an application should be made to the Probate Registry covering the area where your mother lived:
http://www.justice.gov.uk/courts/probate/probate-registries