Clarification on property sale

My grandfather and my grandma has 4 sons and 3 daughters. My mom is one of them. He sold his house for 3 crores. But gave my mom only 5 lakhs. My mom has signed the registration document as she believes in them and she doesn't know . Each share (9 shares include grandpa and grandma) is approximately 30 lakhs. I heard that we have to pay 30% of the share value as income tax, which is 9 lakhs.I don't want their money. But I can't pay the tax from my pocket. How do I defend this without my mom's support?