I've been to a few receptions and let me tell you that the DJ has a huge influence on how things go. I don't just mean the music choices but their skills at the sounds board as well as their communication abilities are very important.
When you are looking for a DJ don't take the job lightly. Your cousin might be good at making playlists but can he run a sound board and be able to adjust the sound on the fly as the surroundings change? Can he make sure you're not getting feedback when someone is making a speech or walking around the room with a microphone? You might be shocked at how much a poorly run sound system can effect a wedding.
In addition to having the basic skills how is he/she at getting a crowd going? When the dancing starts can he get things rolling and make it fun? You will probably have an MC but once the speeches and toasts are over the DJ takes control. You have to have someone who projects fun and enthusiasm. A DJ who curls up in the corner of the room behind his laptop and ipod is not what you need. You need 1-part sound technician and 1-part entertainer.
Here is an article I found on some things to look for when hiring a DJ. Whatever method you use please don't make this decision lightly. This person wields a great deal of control over the day.
Ten Questions to Ask Before Hiring a Wedding DJ