The ornamental design for a sinkassembly, as shown and described.
FIG. 1 is a perspective view of a sink assembly showing our design;
FIG. 2 is a front elevation view;
FIG. 3 is a rear elevation view;
FIG. 4 is a right side elevation view;
FIG. 5 is a left side elevation view;
FIG. 6 is a top plan view; and,
FIG. 7 is a bottom plan view.
The broken-line disclosures in the view represents portions of the sink assembly in which the claimed design is embodied, but which forms no part of the claimed design.