All of the MA timeshares listed are located on Cape Cod. If you opt to stay there so you can drive in to see your nephew, you're looking at a 1.5 hr drive one-way WITHOUT traffic - this time can easily double or even triple during Fri/Sat/Sun/Mon in warmer months.
I'd go with a timeshare stay in Williamsburg and a few days at a hotel on the metro line to DC (assuming you don't own Wyndham points for a 3-night stay in Alexandria on the metro). Invite your nephew to join you if possible. If you trade through II, you might try for the Marriott in Boston and fly down to DC for a few days afterward. Boston is also full of historic sights, completely different from Williamsburg since the city has continued to develop and modernize around the landmarks. I think you'll need to choose between Boston and Williamsburg for this particular trip though.
Combining a visit to Williamsburg with DC is easy and fun - an excellent combination with far more interesting and fun things to do than you can fit in 10 days, no matter your personal interests. Also, if you rent a car for the Williamsburg leg of the trip (highly recommended), you could drop it off at your DC destination. Parking in DC is expensive and hard to come by and the in-town roads are confusing to new visitors. Public transit is very good so use it there!
Combining visits to NYC and Boston is fun too as these are very, very different kinds of cities, both loaded with character, history, familiar sights and interesting, varied things to do. They're only about 4 hours apart by car, avoiding rush hours. Or, take a quick flight between them, since having a car in either of those cities is definitely more of a liability (high cost parking and aggressive drivers) than asset. Again, the public transit in both of these cities is excellent. HTH.