Sale Of Property Without Consent Of Grandson

I have one elder brother and two sisters, all are married. Our father and mother are no more. Our father had saved several assets through his efforts in his lifetime and also has his father's property. No will is made by our parents. Now, we are planning to sell one of the properties with mutual understanding between brothers and sisters. Both I and my brother don't have kids..but both the Sisters have two sons each. Now one of the sons (major) is threatening that, it's his grandfather's property and he has right on it and can put stay for the sale. Pls clarify, Q1 Whether properties has to be equally divided and then grandsons can take their share Q2 Whether before dividing the equal share, shall we sell the property at our wish and grandsons cant interrupt