Update from EWR: I had my interview yesterday morning in Newark. The woman in front of me let the officer know that her parents and sister were with her and asked if they might also be interviewed. He told her it shouldn't be a problem, but wanted to get through all of those who had interviews scheduled for that time slot first. This was different than the experience we had last week when I was unable to get interviewed at my wife's appointment.
Bottom line: YMMV with walk-ins at EWR if you have are with a family member with a scheduled slot.