How to slightly enlarge a worktop hole for replacement kitchen sink
If there is adequate room at the rear of the sink, or if you worry less about centering and just do the sides and the front, a router will do the job nicely. A laminate trimmer may be a better choice on the "fit's the back of the sink" part as they have a smaller base, but they also have more limited bit size/power - still you could do it in a few passes, and they cost less to buy.

The secret to nice edges with a router is to build a fence or template - a board supported the same distance away from the desired cut as the edge of the base is from the bit. Freehanding it will tend to wander quite a bit more than following a fence. Firm control of the machine is also needed, especially when working on a narrow strip of material as you will be in this case, at least at the front.