These dollhouse sinks in farmhouse styles are simply made from a rectangular wooden box, finished with epoxy putty, paper clay, air dry clay, or gesso to give it the look of ceramic when finished with a coat of white or cream acrylic paint and a final glossy acrylic clear coat or a coat of clear nailpolish. The sinks shown on the photo on this page are not finished (you'll be able to see the finished sink on the cabinet base in a later tutorial). I've set this sink up in the standard commercial measurements (29 inches by 18 inches by 10 inches deep) scaled down to 1:12 scale. It is easy enough to scale up to 1:6 scale, or down to 1:24 and 1:48 scale.
In smaller scales you may want to ignore the wooden box, and make the basic box from lightweight card or paper. You can use a heavier cardstock (bookboard or illustration board)to make the larger scale sinks instead of wood if you wish.
Just for comparison, I've shown a variant of the sink in the progress of being formed from polymer clay in a single serving jelly / jam package. If you dust the package with talcum powder you should be able to shape the clay in the mold and release it to bake or cure it. As you can see in the photo on this page, sinks made in these molds are much smaller than the sinks made to the measurements of full size sinks. They may work for smaller scales made this way.