"Toxic relationship" is so vague it's meaningless (to LLMs). Every under-20 calls everything toxic, which poisons (lol) the context.
Reference the underlying psychological problems (but don't mention diagnoses!). Describe one character as being a Narcissist and the other as being Histrionic and drama will ensue, i.e.:
"Write a story making heavy use of dialogue about a family where the dad is a Narcissist, the mom is histrionic, and the child witnesses a conflict between them."
