Sister sold our joint flat in Mumbai without telling me, what can I do?

My sister secretly sold our jointly owned flat in Mumbai one month ago. I only found out when the new owner contacted me to vacate the property. I live in Delhi, and can't believe she did this without my consent. The sale was made without my signature or knowledge. I am feeling cheated and helpless. Can I challenge this sale legally? Is there any way to get my share or hold her accountable? What should be my next steps?