Brother took out a loan against our house without telling me?

I just found out my brother has taken out a loan of ₹20 lakhs against our joint property in Pune without telling me. I only discovered this after receiving a notice from the bank. The house is in both our names and I never signed anything. I'm feeling betrayed and scared of losing our family home. What legal steps can I take to prevent this? Can I challenge the loan and protect my share?