Want To Know My Rights On The Property

My name is Sumit Panja son of late Bharat Panja.My father had two marriage. 1st wife has 2 sons I am one of them & the other one has two daughters. My mother and my brother don't live with us. I live with my father, stepmom & two daughters. My father died in 2002, but before that, he made a 'will' to distribute the property by front half to my father and the other half to stepmom & 2 daughters altogether. We lived by that partition for more than 20 years. But now they are claiming to distribute the property by equal 4 parts. So all I am asking can they claim the property after 20 years. Cause I lived in that area for more than 20 years and according to adverse possession they can't claim any right for the property after more than 12 years, or any other law to get my rights. I will be very thankful if you please give me a suggestion.