This would be a case of think like a carpenter. The carpenter doesn't need the hole to exactly fit the sink, the sink just has to fit in the hole and since it has a return around the top edge, it will cover the parts of the hold that don't match the sink.
The lazy man's way:
First, hide the sink. Double click on the top until you get through the different groupings to where the top most surface is highlighted. Draw a rectangle on the surface of the top. Push pull the rectangle through the depth of the top. Turn on Hidden Geometry under the view Menu without exiting the sink top group, you will now see the ghosted view of the sink. Using the move tool or the push pull tool, move the sides of the hole until they are under the apron of the sink edge. Once that is done click out of the group, right click on the hidden sink and unhide it, turn off the hidden geometry under view and you are done. This uses very little geometry and represents what the carpenter will really do (they may put a radius in the corners but not always).
Hope that was clear.