Personally, I do not go to the parks on my arrival day. For me, I like to experience the opening show, and then the made dash (or quick walk as it is now) to my favorite attraction. On the first full day of our last trip, my husband and I started off at Animal Kingdom. We were there early to see the show, and then immediately went off to Everest. We ended up being in the first seat, on the first ride of the day. For me, it was the absolute perfect start to our vacation!
Usually on arrival day, we check out the resort, eat in the restaurant there (if applicable) and sometimes go to Downtown Disney.
Since your sister and family are first timers, I would suggest making Magic Kingdom your first park of the trip. If your nephews/neices are into thrill rides, start with Big Thunder Mountain and Splash Mountain, then head over and grab a fast pass for Space Mountain.
Just my thoughts anyway! :shrug:
Oh and have a great time!!