how to proceed the case without wakalath

on behalf of my mother through GPA, i was handling partition suit in the court, recently my mother expired, left with registered will and my mother had 3 children. Whatever property, has been divided equally to me and my brother. Now proceed our case further, previous GPA got void, need to be submitted wakalth form to the the court, my sister unwilling to sign the wakalth form, without her involvement how to proceed the case legally further, kindly answer to my query. thanks