How to I respond to my brothers ultimatium

My brother has fallen under the trap of my Nparents. My parents don’t like I’m dating someone out of my culture. They have said so much HORRID shit to me and my brother who is SIX years younger than me texted me and said if I don’t break up with my boyfriend by his birthday (June) then he’s going to disown me as a sister. I never responded and today he texted me saying (a few days) left. What the FUCK do I even say to this shit. I’m so fucking angry.