Final answer:
A nurse should suggest the National Alliance on Mental Illness's family education series to a neighbor dealing with a sibling's bipolar disorder for additional support and strategies.
Step-by-step explanation:
The best response for a nurse to provide to a neighbor concerned about a sibling with bipolar disorder who refuses medication might be: "The National Alliance on Mental Illness offers a family education series that you might find helpful." This response is supportive and provides a resource that could offer additional strategies and support for dealing with a loved one's mental health condition. It encourages the neighbor to seek further education and support, rather than suggesting drastic changes to the sister's treatment plan or advising the neighbor to abandon the relationship with their sibling.