Thing is, it's for family - which may change things... Our session fee is $275, but I'll waive it for family. Others will tell you to never treat family any differently from a normal client.
Also, what are you charging for prints? Your normal rate? How high is that? If you're being incredibly generous with your print prices, make sure your get enough of a session fee to make it worth while. If your charging a repectable amount for prints, then maybe you just want to charge a token amount so that your Uncle doesn't feel like he's taking advantage of you...
Sorry, a lot of considerations. And a lot of it has to do with you and how you want to handle it. We're doing a wedding for an uncle next year - we're charging our actual travel expenses, a modest amount for our time, and providing most products at cost. Of course, I blew up his van once (no kidding

) and would have done it for free - but he insisted on paying.