If you know where your deceased sister lived, there's a good chance that she was cremated at the nearest crematorium to her home so if she was cremated in Brighton, for example, you could try Googling "crematoriums within 30 miles of Brighton". This might give you a list of them and then it would be a matter of ringing them all up and asking the Administrator (or whatever they are called) if they have this information. (If her ashes were scattered in their Memorial Gardens. Having an approximate date of death would probably help in this search. The other thing, if you know the place of her death, would be to contact any local papers in the area to see if there was an Obituary or Death Notice published which provided details of her funeral arrangements. You would probably need to contact their Archive Department.
If her ashes were scattered in some other place I think it would be difficult to track down any plaque.