My cousin sold our joint property in Mumbai without telling me, what can I do now?

I just found out that my cousin sold the plot we jointly inherited from our grandfather in Mumbai. He didn't inform or take my consent. The sale happened two weeks ago and the new owner is already trying to take possession. I'm feeling utterly betrayed and don't know what to do next. Can I challenge the sale in court? What legal actions can I take against my cousin and the buyer? I'm really worried about losing my share in this property.