Right to ancestral property even after giving release

My father has two brothers, one elder and the other younger. The property was in the name of our fore fathers(some three four generation back). More than 20 yrs back (start of 90's) my fathers elder brother gave an assurance that we will make the property in the name of small brother and they would partition the property later. Basis this assuarnce my father gave a release in property. This release was taken from all those who had the right in the property and the property was made in the name of my fathers younger brother. My Father had given release from ancestral property in the name of his younger brother 20 yrs back only on the assurance that he would get his share in the property later on. My father wanted to build a house in the ancestral property in early 2000 but before he could do anything he passed away in 2004. Both the brothers (still alive) knew this fact that my father wanted to build his own house in the ancestral property and the younger brother says that he will give my fathers share to us (but is making excuses to give the share after a few years) when i asked my fathers share, but as my father has given the release not sure what options are available to me and my brother/sister to get our fathers share of the ancestral property. what if my fathers younger brother denies any right of us in the property what can be done by us in such a case. Please suggest.