I'm not understanding if you're in the Chicago area or if you live in another city that your sister would have to drive, too? If you live in Chicago, I think it would be reasonable to expect to see your sis before she leaves. Whatever way is best for her, i.e. you take lunch to her and help her pack, etc or invite her over for dinner.
If you live in a different area from your sis, then I can see how it might not be possible to see her before she goes. They say a move is one of the 5 biggest stressors in life.
OK, so I just re-read your post. You are getting to see your sis, just not your nephew? They pick him up right before moving, so it would seem that timing-wise it's not possible to see him? Maybe you could plan a trip to see her and her family when they are settled?