My cousin sold our joint property in Thane without telling me. What can I do?

I recently found out that my cousin, who co-owns our family property in Thane with me, sold it without my consent. I never signed any documents and was completely unaware of the transaction until a relative mentioned it. This has left me feeling deeply betrayed and unsure of my legal standing. The property is valued around ₹80 lakhs. What are my legal options to challenge this sale and protect my share?