Cancellation of will registered by court on inherited property.

My grandfather has given a will to my stepsister stating that she will be the rightful owner in absence of my father as his elder son has immigrant to other country and father is living with another wife in another state but will clear that if my father comes back he will have his share and one share is for my stepsister and one for my uncle who might be back incase.but my sister refuses to gv the share of my father as a result my father is mental disturb and becamemental paitent aftet 14 yrs i got to now the story and want to move to the musiff court whr my grandfather registered the will .but some are saying that i cant fight on property whn my father is alive.can i take this with court if yes can i cancel the will as it is inherited property and if my stepsister get share and why not my father sisters will get it from it .what documents i have get ready for my case to make it strong