Since you don't know why your brother suddenly cut you out, do you know for certain if it has to do with your cousin, or perhaps it's other issues between you? Do you have other issues or points of contention between you and your brother? Has he brought issues to your attention that you've perhaps forgotten about?

You have every right to your feelings about your cousin - it's not your brother's place to dictate your relationships.

I would just stop responding to your cousin. No need to close your Facebook or block. I just would not reply. Only my opinion of course!