The rule of thumb answer would be to include the things by default which make sense, are helpful to your consumers, and don't bloat your responses beyond your idea of a reasonable limit. To answer your question specifically requires visibility into consumer needs, domain model knowledge, caching strategy, deployment strategy, etc..
A relationship is by definition bidirectional. However, it doesn't mean it is always explicitly defined, or stored in a way which is easily referenced. Speaking in terms of traversing a graph, two points connected by a directed segment don't somehow become unrelated in the context of the target point because of the absence of a traversal path to the source point.