Dad purchased plot years back now children of seller threatening

my dad purchased a plot area from a lady before our birth half area of the plot dad constructed our small house as we all are settled in mumbai we rarely go native now after more than 30 years some people have kept eye on that remaining area as we are planning to construct remaining portion through house scheme that old lady son is creating problem and threatening us that we will not let you all survive here.and also saying tht you have not taken signature of his sister. But this property was purchased when we werw not born and infact they were also minor . along with our two uncles also purchase plot with us and they also constructed two big house no problem he create.only threatening my dad saying I will not let it happen will put stay order on saying of few villagers.sir please advice