Sale of ancestral Property

This is regarding sale of agricultural land and residential land by my aunt. Both land (located in Munger Bihar) is in the name of my grandfather who is no more. My grandfather has 03 sons. Land is undivided and without demarcation between my father share and uncles share. My uncle (a govt servant) has no child from his first wife therefore he got married again. My uncle got is settled in Bokaro Steel City. Second marriage was not legal , without intimation to govt office. My aunt is 2nd wife of my uncle and she has 03 daughters. My uncle expired few years back without any legal will. Now my aunt wants to sell her share of agriculture and residential land to someone else. I asked her to sale to me at the same price she is getting from others but she refused to sell to me at the same price. She is demanding more me as compared to others due to previous family disputes. I have fear that some other person will enter in our residential area. She is threatening to sell the land to third party. Therefore, it is requested to suggest me the possible legal ways to stop her to send the land to third party.