Anna's ex-boyfriend has mentioned that he would have been on the cruise if they hadn't broken up. I wondered at the time if it was hopeful thinking, but more recently have wondered if he was actually part of the original plan. Cruises are often planned many months in advance. If the original plans involved the ex-boyfriend, a likely arrangement would have been younger children the (step)parents, the three teen males in a separate cabin, and Anna staying with grandparents. Something many of us would have seen as a rational arrangement. Possibly, when the ex was no longer in the picture, moving Anna from the grandparent's cabin to the teen cabin may have been seen as a way to give her a full bed.
From what we have now heard, I don't think stepbrother and boyfriend in the same cabin would have been a happy combination.