As an NES (UK),but not a teacher, I would explain the difference as follows:
If something is "relevant" in a discussion, it should positively contribute to the discussion.
If something is a "related" matter, then it might cover a similar, but perhaps different situation, and may not be relevant to (i.e. not apply to) the particular discussion that you are having.
Objection: Calls for an irrelevant answer Legal Basis: The answer to the question would not make the existence of any consequential fact more or less probable.