Cousin sold our jointly owned land without informing me, what can be done?

I am from Hyderabad and my cousin sold a piece of land that we inherited together from our grandfather. He never told me about any sale and now the buyer is claiming ownership. I found out only after I visited the site last week. I feel completely betrayed and worried about losing my share. Is this sale legal? How can I challenge this transaction and ensure that my rights are protected?