If you're coming down 128 you can either get on the Mass Pike and get off at route 30 (Comm. Ave.) or you can take 128 to route 9 east, turn left on Hammond St. and end up right at the campus. I think route 9 is probably easier.
I don't know how much time you plan to spend there but generally the the nicest area is the middle campus. Definitely check out Gasson Hall, Bapst Library, Devlin Hall (McMullen Museum of Art), Fulton Hall and Higgins Hall. Both Higgins and Fulton have beautiful atrium's where you can sit down and take a break if you want. You may also want to stop by the bookstore in McElroy.
You should also take a look at O'Neill (the main library) and then walk through 21 Campanella Way to the lower campus. There is a small bookstore and a cafe on the first floor of Campanella. On the lower campus you could check out Corcoran Commons (the largest dining hall), the Mods, and the New Yawkey Athletic Center (They have Fluite's Heisman Trophy in the lobby of Yawkey.)
If your sister wants to check out the freshman dorms you should do so after you visit the Bookstore in McElroy. They are across the street at the upper campus - Gonzaga, Fitzpatrick, Cheverus, etc. There are also freshman dorms on the Newton Campus (about a mile away) on Centre Street.
Before you go, get something to eat at either Pino's Pizza or Eagle's Deli in Cleavland Circle (intersection of Beacon St. and Chestnut Hill Ave.).
