Can HR terminate me for not disclosing my relationship with colleague?

I have joined an organization where my brother-in-law also works, but he hasn't disclosed our relationship. Somehow, it came to the management's notice after 16 months. I went through a background check, everything went well, and I was offered the job. I have been rendering my services for the past 16 months and have been a top performer. My brother-in-law is neither my line manager nor my reporting boss. Now, the HR is threatening me with resignation or termination. Please advise me on the way forward.