Is it possible to make the kitchen sink the most important object so that it basically cuts a hole in the bench when it is placed there?

It's not a problem of what is more important to other objects. Sweet Home 3D is simply unable to compute holes of an object in an other one. The only holes that can be automatically computed are the window and door holes in walls and the staircase holes in floors.
The workarounds for your issue are to use a sink 3D model included in a cabinet, to place small thin boxes around the sink, or to create a customized 3D model with an other software like Art of Illusion.