In Opal, the hierarchy among topics is described metaphorically in terms of generational relationships among females.
Thus, one topic is the sister of another if they are subtopics of the same topic (that is, they are daughters of the same mother).
Topics are ordered, meaning that sister topics are ordered among themselves. Thus, in the sequence of sisters, a topic may have a previous sister (its immediate predecessor in the sequence) and a topic may have a next sister (its immediate successor in the sequence).