I traveled to WDW earlier this year with my family and met my parents there. My parents were staying under a separate reservation. We made a request to be placed near them in the hotel and, as we asked, they were given the room directly across the hall from us.
Making a specific room arrangement request such as yours is a two-step process. The first thing to do is call Disney reservations (407-W-DISNEY) now to have your request added to your file. It doesn't matter that you are traveling under separate reservations, just give the reservationist the names of the other members of your group and your request for connecting rooms. You may also want to have your sister and parents call to make similar requests so that the notation is on everyone's reservation.
Then, you'll want to follow step three on Bret's answer about room requests
HERE. Confirm your request with the hotel directly about a week in advance of your arrival. If it is at all possible to accommodate you, Disney will.
Have a wonderful visit to WDW!