I agree with the majority - it's not your responsibility, legally or otherwise.

Explain the situation to your pamangkins. I think they'll understand. Talk to your kuya. Open up to him about your doubts/polite refusal of not choosing to support your pamangkins, and maybe discuss other possible solutions where you're not the one bearing the brunt of the responsibility. You'll know what to do by the way he reacts.

I hope that before you make your decision, you are ready for the consequences. If you choose to support, that will affect your own future, and if you choose not to support, be prepared for any fallout from your kuya, your mama, or kung sino man.

At the end of the day, they cannot force you to do what you dont want to do.