I just listened to the video posted above. My family was on this very ship the week before this incident. We were a party of 12 (6 cabins) one floor above (we were deck 10). To explain the layout of the ship, at least in the center of the ship where we were (they were in the rear of their deck) deck 10 balconies stick out a bit (half covered, half uncovered) but look down into the balconies of Deck 9, which stick out further (all uncovered) to cover the lifeboats below, on deck 8.
https://www.google.com/search?q=vie...&biw=1346&bih=689&dpr=1#imgrc=krl5C3DljJsWzM: That said, the lifeboats don't go all the way to the front or back of the deck, so where their rooms were, there were probably no lifeboats below
https://www.google.com/search?q=vie...&biw=1346&bih=689&dpr=1#imgrc=cfnNXP_tBZRH3M:
They were in the last few cabins in the back, on the side, so it does look like Deck 10 balconies overhang deck 9, so people above probably could get a view of what was going on
https://www.google.com/search?q=vie...&biw=1346&bih=689&dpr=1#imgrc=qKPQQScUi6DVVM:
This may be a better view.
https://www.google.com/search?q=pho...#imgdii=Q6An7SbtsDbzlM:&imgrc=xw4TwLDhsxDWPM:
And as the audio explained, the cabin doors lock when they close into the hall, BUT, if you wish, you can ask the steward to unlock the doors BETWEEN the balconies (which is what we did) (there are doors in those white dividers between the balconies)
https://www.google.com/search?q=pho...#imgdii=Q6An7SbtsDbzlM:&imgrc=xw4TwLDhsxDWPM:
Here is a photo of the balconies with the door between them open
https://www.google.com/search?q=pho...&biw=1346&bih=689&dpr=1#imgrc=w1yigvPbtP4n-M:
So I am guessing the bro in law and his family had that door unlocked between the balconies so the families and kids could go back and forth and hang out on the balconies, and this would be how (again, I am speculating) the family members could get into the cabin when the girls ran out the front door and it locked behind them (as it is supposed to do). If you close that door very gently, you can keep it from closing all the way and locking, but its doubtful in their panic that they did this.