The ornamental design for a sandsifter, as shown and described.
1. Sand sifter
1.1 : Perspective
1.2 : Perspective
1.3 : Top
1.4 : Bottom
1.5 : Left
1.6 : Back
Sand sifter for use with a cleaning bucket; front view is omitted because it is identical with the back view; right side view is omitted because it is identical with the left side view.